Since the new alternator uses different connections (screw terminals instead of a three pin socket), I had to replace the wire, so I used a 16mm^2 wire and connected the other end to the battery, which is quite close to the alternator. The wire drops about 0.03-0.1V (IIRC ~5A current results in ~7mV voltage drop).

The original belt was good enough, the only problems I had were getting the "gear" screw out (it was quite wedged in there) and getting the new alternator in the bracket (it probably was just a little bit thicker than the old one).