Dan's (www.falparts.com) Imbel kits vary from very good...little finish left but mechanically fine (Grade III / $129)...to almost new/unissued (GradeI / $250). Excellent people to do business with. Imbels are Brazilian-built on FN machinery and have chrome lined barrels. Most are 1970's or 80's vintage. They exactly match the Imbel receivers being sold by SARCO, FAC and others as to "correct" type.
G1's are old W.German service rifles...all built by FN between 1958-1960 (I believe...if that is wrong, someone will correct me I'm sure :-) and then sold to Turkey where they pretty much beat the shit out of them, but seldom shot them. G1's do not have chrome lined barrels and were issued with wood buttstocks and pistol grips. It is more difficult to find a really nice G1 than an Imbel as many of the G1 barrels are pitted and there can be some problems with gas plugs. Originally, however, the G1 was made with more attention to detail, etc.
There are no significant differences in assembly of either kit, but the consensus among the build-it-yourself FAL crowd is that there are more likely to be "issues" with G1's than many other kits commonly available right now. I have both, and chose the Imbel kit to have a new 16" lightweight carbine built up by ARS. I have a pretty nice G1 kit, but I may sell it, or, may just try and build one myself on am Imbel receiver.
I am NOT an expert on FALs, but maybe this will get you started. The FAL is one Hell of a rifle and well worth spending some time and money to get a good one.
Good luck.