AR 15 kits
If you want to go the cheap route check out SARCO I've built some from them. But the first thing you should do is to go on line to You Tube for a video on building an AR15.
Delton Kits
I have used DelTon and never a problem most of the problems that happen with any kit is that the assembler does not know what he is doing. The AR 15 is a sophisticated rifle and you really should know what you are doing.
AR 15 kits
I have bought a few from SARCO and always have been satisfied. I usually build them to early carbine and rifle specs. My receivers usually are from either Superior Arms because I like the trigger adjustment or from Bushmaster.