It should be a lot easier to find a 20" AR now than a couple months ago. BCM has nice 20" uppers. But you should answer SSGN_Doc's questions, we can only give generic answers to a vague original question.
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