Honestly, if the end goal is to get a mac, and this machine doesn't have to pull double-duty as, say, a gaming PC, then just get a cheapish mac-mini and max out the RAM using a third-party source (Such as OWC). A reasonably configured Mac Mini is going to be not much more expensive than a reasonable spec BYO PC, and you don't have to deal with the headache of potential parts incompatibility or DOA parts having to be RMA'd before you have a working machine. Granted, such things are rare occurrence, but it can really take the wind out of your sails when it happens.
Then, consider that you might have to re-buy or re-license your software when you switch platforms, in addition to learning a new working environment and its associated workflows -- I suggest you'll be better off just starting with a Mac if that's the end goal.
If you live in/near an urban center, your local craigslist probably has a number of mac minis for around $500 (I see these all the time near Seattle) that are upgraded beyond the current least-expensive retail spec. The only minor caveat I have, is that if you do buy a new retail unit, its suspected that a haswell-based refresh should be happening before the end of this month, so you may want to hold off until then. I also see plenty of Xeon Mac Pros of various vintage sometimes for sub-$500.