Saleen,
I don't have answers to all of your questions, but here is what I can tell you:
1) The FEG is a properly manufactured rec that is hardened in all the right ways, and has the nice X and Y stamps over the FCG pivot pins however it is more expensive and doesn't count toward your US part count. The OOW rec is by most accounts adequate for building up a kit but does lack some of the FEG's above features. Other rec options: Global is one day (allegedly soon) going to have a complete US made mil spec rec, but as of now is still vaporware. However, all of these stocks are designed for fixed buttstocks and would require quite a bit of work to attatch the folding stock.
2)For a stamped reciever rifle you need to add 5 US parts for compliance 6 if you're adding a foreign made muzzle device. The most commonly replaced parts are:hammer, trigger, diconector, gas piston, pistol grip, buttstock and hand guards (2pieces but only count as 1 part). You can also replace the magazine followers and floorplates, but that means drums are out of the question and you need to make sure you keep your magazines straight. I don't know personally but the K-var and Global US FCG are about equal and most will tell you to live a little and get one of RSA's excellent adjustable trigger groups.
Unfortunately thats about the limits of my knowledge. I am sure someone else like Campybob can give you the lowdown on welding the stock and pressing the barrel.
Good luck and happy building.