Siennfein,
Whew. You are looking for an indepth education in a short time. Look, for your first AR the important thing is to buy one of the recognized brands so you don't have problems. Colt, Armalite, Bushmaster are all quality firearms. If you buy new, IMO, the Bushmaster is the equal or better of them at a competitive price.
Now on the Colt, IF it is truly a preban rifle, original, with only 20 rounds fired (and not rusted shut or something) then it is a bargain at $1000 and YOU SHOULD BUY IT. Why?
Well, even assuming you don't want the evil preban features (but you will in time), the resale will remain in the preban Colt over the new BM, etc. For example, if you buy a new BM for $750 today, next year it is worth about $600-650. Buy the Colt for $1000, it is worth $1000 in future - or maybe more (unless you mess it up).
The scary thing is, you still haven't identified the model Colt this is. SOME preban Colts are worth a lot more than $1000 in that condition. I have seen Colt lowers (just the lower reciever, handle and buttstock) sell for near $1000.
The only danger I can see to buying the Colt is if it is not in good condition, or it really isn't a preban. Get the model number and serial number and you can look it up to verify it's status.
Good luck.
James