I was in your position last year. I had never built one before either.
I bought a DPMS lower, sat on that for a bit, then bought a lower parts kit and installed it into the lower with the help of the online assembly video from Brownells. It wasn't difficult at all, just take your time and use good tools.
Then I went to a gun show and picked up an M&P upper with a full length rail second hand. It was a former PD trade in I believe. Added a Rock River stock, a carry handle rear sight and a nice front sight from someone on another forum and that was it!
This was before the price explosion, I probably got $800 into it.
I love the M&P, Here's the rifle that the upper was from.
That's a Troy rail system, which is real nice. The M&P has the heavy M16 bolt, chrome barrel and is a flat top receiver.
Here's what she looks like now.
I may add optics later if I win the lottery, but for right now the iron sights are fine. I've only shot it at 100 yds since that's the farthest my club's range is. I shoot very tight groups consistently with it. Problem is ammo's so expensive right now, so until I can get some small rifle primers it sits in the safe