Welcome to the site (and the addiction)!
Just my .02:
Stay away from cheap, cast ARs. Cast receivers are (/can be) weaker than forged. You should be able to build a lower for, well let's see:
Stagarms stripped lower receiver from
Eagle Firearms for $79, plus S&H (say $6) plus FFL transfer fee (say $20) is $105 to your door (so to speak).
A Rock River Arms lower parts kit is around $54. You should be able to pick up a used A2 stock kit on the EE; estimate $45 to your door.
So, for sake of simple math you can get into a QUALITY lower for ~$205.
Then get a complete upper from any of the site-supporting vendors (Pete in NH, Eagle Firearms, ADCO, CMMG, GA Precission, etc.) and you'll have a QUALITY firearm. You can get nice RRA upper for $375 and up, depending on what features you want.
Look, this might be a little more than the plinker you saw, but if it were me, I'd rather spend just a little more on a QUALITY firearm that is going to be dependable.
Hope this helps.