In a ban state if you don't know all the ins and outs of your laws, and I mean ALL, then it's not wise to do your own build or buy used.
Since you're going cheap it's hard to beat a compliant M&P-15 Sport at your LGS since they better know the laws. If you buy new online (and PSA is a great way to go) the liability falls on you to buy a compliant AR.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek