CETMEs are just cheap Spanish clones of HK G3s, so they suffer from all the problems of the $2500 G3s plus lower quality control and poorer fit and finish.
CETME mags are cheap and plentiful (usually under $10 ...
here's 10 for $45)
B Square makes a G3/CETME scope mount that can be picked up in the $50-75 range that I hear works well.
IIRC the biggest complaint about the G3 is that it destroys brass.
That said for $350 I don't think you could go wrong ... just don't expect it to perform like a PSG-1. But if you find you don't like it you'll be able to get more than your money back out of it (even if your friend doesn't take it back).