Well I think I just fixed it and although it's a little embarrassing here it is in case it helps someone else
The buyer didn't go ahead with the deal because we took it to a local shop for inspection and the charging system voltage was a little low (high 11s to 13 volts) so I started suspecting the usual things: the rotor because the previous owner had dropped it on that side, the stator because the service manual test measured 0.5 Ohms (although VAC was good at ~54V) and the regulator. So I removed the cover for inspection and both rotor and stator looked ok. In page 7 of the burnt stators thread onewizard confirmed that most ohmeters aren't precise enough to carry out the service manual test so I didn't really have a reason to suspect the stator anymore. The battery was a little low (12.2V) but I had it checked and it was ok. The regulator passed the continuity test (didn't do the other tests because I didn't have enough batteries). Checked the chassis ground by the tail lights and it needed a little cleaning up but nothing that would make a big difference.
And then my brain finally turned on. Idle speed was at ~1000 rpms. The mechanic had said that was ok so I forgot about it. Idle speed should be of course 1300 +/-50 rpms per the service manual and as soon as I adjusted it the system voltage went up to 14.1. Now it's hovering between 12.8 (with high beams, hazards and rad fan on) and 14.1 V, assuming that's an acceptable range. The strange thing was that even when set at the lower idle (1000) it would still have low voltage at the higher rpms (<3000).
Hope that can save someone as much time as it took me to figure out
Last edited by jcsantisteban; 11-11-2015 at 12:22 AM.