I personally have had excellent results building on Imbel kits... I also did a build on one of the R1 kits currently being offered by the Ammunition Store for $150 - it turned out OK, but the kit overall wasn't quite as nice as the Imbels I had already done... Gunthings is a decent outfit to buy from, too - I think either option will work out OK for you...
"What are the common parts for replacement for 922 compliance?"
I typically go H,T,S(DSA), receiver(DCI), PG(Wolfsburg Bob's or SAW), muzzle device(FSE), AND floorplate(heddonpal) - for furniture, I prefer the Imbel fiberglass handguards, and STG-58 buttstocks - YMMV... Many people replace the gas piston, or buttstocks - as an option, too..
"Better to buy them single or buy a kit from Tapco of someone?"
Depends on what you want, and what is the rattiest in your kit that can be replaced to make compliance...
"If I use a mag floor plate as one of the US parts, technically do I have to have one on every magazine I use?"
Well, if you don't, then you *could* run into a problem if the "wrong" mag is in your rifle at the "wrong" time... US floorplates can be had for under $1 apiece, so I just replaced them on all of my mags - they look good, and are easy to replace... I take that opportunity to clean out a "new" mag thoroughly, and slap in a new US made floorplate - it's not a bad route to go, IMO...
- georgestrings