I don't have any direct experience with Azex, but I've heard nothing but good things about them. They use Austrian STG58 metric parts kits on metric Imbel receivers. Metric mags are cheaper and more plentiful, as are really good-quality parts kits.
Mark Graham at Arizona Response Systems also builds good FAL's, and he will build inch or metric, I believe. I just ordered an 18" metric FAL from him, so we'll see how it is. I decided to go with him because he offers the shorter-barreled guns, which I prefer. Azex might be willing to build a shorter-barreled rifle, but they don't list them on the website. I also don't like the imitation STG muzzle brake Azex uses, but again they'd probably put whatever legal brake you wanted on your rifle.
ARS makes some really nice-looking 16" and 18" FAL's, and he makes them on Imbel or DSA receivers. Either one will serve you well, but I was willing to pay the extra hundred bucks for the DSA, 'cause I think it's a little better than the Imbel. Take a look at www.arizonaresponsesystems.com.
Definitely stay away from Century and Hesse. I suspect Azex and ARS make equally good products, and their prices are similar.