Hey.
A complete forum-noob here, but I've been using FL Studio since version 6, so at least I feel qualified enough to give my input on that matter.
(Though it's been pretty much a month from the last post, I hope there's still something useful in this).
1. Addressing your first question, yes, FL Studio does have everything a beginner would need, though so does pretty much any other DAW.
FL does have a bit of a different learning curve from the others, so I'd just recommend getting the demo (full-featured sans the option to save project files) and seeing how it clicks with you.
2. When it comes to the range of default virtual instruments, it depends on what you're looking for. It comes with a reasonable range of VSTi in terms of synthesis; the native effects plugins are also more than sufficient for the beginner.
3. Expanding your instrument collection isn't difficult per se. Though it can be an overwhelming process at times, since the selection is just enormous.
Again. Depends on what you're looking for.
3.1. First off, there are Image-Line's own VSTi and sample packs available in the IL store.
3.2. Secondly, the internet is full of third-party software, ranging from full-out orchestral instrument libraries to little softsynths designed to make some specific sound, whatnot.
The price ranges from free up to well into thousands of dollars. Though you do get what you pay for.
4. I can't offer a truly objective comparison with other DAWs due to the general lack of in-depth experience with them. But I definitely agree with Nate. Try out a bunch of DAWs and stick with what you feel most comfortable with. Once you get the basic hang of 'what's this knob do,' it's not difficult to branch out to using other DAWs if needed.
(4.1. FL Studio does have a certain pseudo- bad reputation of being a "fun little toy" / "not a legit DAW". The history behind that being that it started out as a small drum machine/sequencer called FruityLoops and is, regretfully, in some cases still regarded as such, despite having evolved into a considerable workstation over the years).
5. Lastly, with soundfonts being mentioned in earlier posts, FL does have a native SF player, though unless you have the Signature Edition, I believe it's necessary to buy it separately to unlock the demo mode. Though for what is worth, I'd recommend Plogue's Sforzando for playing soundfonts. (Free VST).
Hopefully that didn't come off as an 'advertisement' of any sorts.
Though I will say that no DAW is superior in every sense. It really depends on the individual needs of the user. I will stress again, that the most important step in picking out your first DAW is to try out as many different ones as possible and sticking with the one that best suits you. You'll likely stick with it for quite a while.
It's kind of a strange type of relationship actually... (The more analogies that come to mind, the more morbid it gets, so I will not dwell on it any longer.)
Again, hope this post was/will be of any use to anyone, especially the thread starter.