If you want to put your own together, there's literally tens of thousands of configurations you can do, but since your looking to keep on the lower end of the price scale you've lowered that number a lot.
You could get an actual parts kit, need to mount the barrel and do some work to it, a few venders have them.
Colt M16A1 Parts Kit, w/ new 20in barrel. Very good to excellent condition.
Or you could got a kit that has the upper already assembled and you just need to do the lower.