1. You don't need to be the most competent programmer nor the best 3D artist to produce a workable prototype of your idea... as long as you can live with compromises. Your game will probably be filled with placeholder art (colored cubes and sphere worst case, lower poly and simpler versions of the target art assets best case), and you will have to make do with 3rd party, copy-pasta and simplified code, and maybe probably worse than expected performance.
But you will be able to test your game idea, and continue refining the game design this way, which is also extremly important. What you have now most probably is not fun at all when implemented into a game... you will need to create a prototype and iterate unto until what sounds like a good idea to you now actually turns into a fun game.
2. You could go for stock art and already existing code, like what the guys before me have already written. It is a good way to hack together a somewhat better prototype, and save some time at some point in exchange for more money spent.
Just make sure you don't expect to be able to build the game you think of with stock art and third party code. You might get close, but stock never is exactly what you need/want, third party code will never be the most efficient way of solving your coding problems, and in the end you will have to either hire someone to touch up the art and code (provided you have bought the correct license) or need the skills to do it yourself.
Frankly, I never bought any stock art and never used any thirdparty code that I didn't want to tweak in the end. I most of the time had the necessary skills, and it did save some time in some cases (like having a base heightmap to work with that I just need to enhance with more details, or just tweaking the colors of a color texture instead of having to create my own from scratch), but if you don't want your game to look like a rag-tag assortment of textures and models from different sources, you will need to be able to tweak what you buy.
Same with code. Often it was brilliant code, with just a feature missing. Sometimes I had a friendly dev of the thirdparty code that implemented the feature for me. Sometimes the idea behind the code was good, but when I pulled it apart the code itself was crap (thankfully in case of Unity most thirdparty assets give you access to the code as the C# scripts come uncompiled), and at least in one case the code was crap AND I had to reengineer it because my use case was much more complex than what the code was designed to handle.
Of course, YMMV. I know many have built good games that they were able to sell with stock art, and many games (like the myriad of RPGMaker "clones" (though I heard some of these actually are very decent RPGs)) use the same third party code to produce marketable games.
Depends extremly on your expectations, and needs. If you don't expect too high quality, the mishmash of styles that results from using stock art from multiple sources might be okay. If you don't need too much assets, a single stock art source might provide you with all you need, leading to a more consistent style. For some assets, the fact that they look rather generic doesn't matter (a tree should look like any other tree in 95% of cases, while your main character should look more unique).
Still, you shouldn't trust stock art and third party code to do all the legwork for your game. Having access to custom art and code, either done yourself or by freelancers, will do a lot to raise the quality of your game.
3. Your plan being realistic or not depends entirely on your expectations, especially when it comes to time. Can a lone wolf create an RTS with decent graphic, a decent singleplayer campaign, good gameplay mechanics and enough unit variety? Yes of course... if the lone wolf brings the skills needed and is ready to invest many years of his life into developing the game.
You can cut down on the amount of skill and time needed by investing money, but with the budget you have I would be hesitant to spend too much, besides for licenses for 3D tools or hardware needed. You don't want to work with freelancers before you have a very good understanding of what you want and CAN build, in what timeframe.
AND you need to be aware that RoI is not guaranteed at all. Investing into game development is like the lottery. As long as you don't have years of expierience and the brand and resources to play in the big AAA leagues, at least.
For the small time Indie, your first few games will be an extreme gamble. Will you be able to get enough visibility to sell any copy? Will your marketing plan be the right one? Is the game you build of enough quality, coming out at the right time, will you find a community?
If I would have to advise people on sane investments, investing in games would be on the bottom of the list. Especially when it comes to Indie games.
Now, with that out of the way, what can you do? How realistic is your idea?
3a. Get the skills needed to at least produce working prototypes. Really, you will not be able to a) finance the game you want to build, or any other game worth building (you might be able to pay for the art and code of a flappy bird clone, and if you come up with a better plan than "yet another clone", you might actually produce quite a good mobile game with that, so take "not worth building" with a grain of salt), and b) assemble and keep together a team of volunteers that will help you build your game (you will need a lot of expierience and a good project that goes according to plan to have a chance of this succeeding).
As long as you are a no-name, with a non-existant budget, no expierience, your best bet is to learn as many different skills as you can and try to come up with good prototypes. IF these prototypes really rock (you know, the art might be not the best, and the code might be laggy at times, but BOY is that a great game mechanic!), you might find it MUCH easier to get help or money in the future when you show that prototype. Not the least because you have shown you yourself believe enough into your own project to go through the hassle and invest the time to learn basic game dev skills and actually build a prototype.
Showing dedication is never a bad sign. And the prototype will certainly help in communicating your vision
3b. It is realistic, if you can stomach the long time needed to make it a reality.
We are talking years to become a competent 3D modeller or a good programmer... even becoming halfway decent in BOTH professions will take you 1-2 years at least, depending on how much time you can spend on it.
Then, building the prototype most probably will also take years... might be months, if you have come up with a good way to speed up the 3D modelling process for the proxy art, and find good third party code or an engine that can do some low level plumbing for you.
Then you will have to market your prototpye, find money, volunteers, or both to help you build the actual product. And then build it. Depending on how good your idea is, and how much people like it, that might take more or less time. Lets say 2-3 years at least.
But really, before you can even start talking about creating a 3D RTS, you need expierience, in all fields involved. Even if you would like to be the game designer, you will need expierience in the other disciplines (because you are kind of the project lead, and your discpline influences all the other disciplines)...
And even more so if you are a small Indie shop, even without being a lone wolf.
So really, start learning today and create small games. Learn about game engines and how to use them. Learn 3D modelling basics, some coding, get a feeling for what is needed to create a game and the time needed.
Then, in maybe 2 years, you are ready to work on your prototype.
Is it worth it? Depends on you. Do you really want to make that game a reality? Or do your really want to be a game dev? Are your ready to invest most of your free time for years in order to be able to say "I created a game, and you can download it on Steam"? If you always dreamed of creating your own game, and you don't care about the cost in skill, time or money, then wait no longer and "just do it"!
If that is not worth it for you, if you are looking for money, fame, or something less time consuming, game development might not be the right thing for you. There are much easier ways to get money or fame, and while building smaller, less complex games might be less time consuming, they also use up way more development time than people would think.