Really depends on your budget. Since you can now find quality 2nd hand AR's on the net from $700 and up, I'd say you can buy one and be just fine. Now, if you have a baller budget and want the mack daddy, then you can buy one already made well into the thousands or build yourself one for about $1k.

Personally, I'd buy one first, customize it as I want, and then if I wanted to step up even further then build it.