I have reviewed all those alternatives, what I have found out so far:
* Konica Instant Press seems a little better than its FP-1 counterpart (better lens, and better build), maybe built for journalism more than anything else, but that's quite alright, great camera; and probably somewhat straightforward to service.
* The 600SE (or its mamiya counterpart) is a fantastic setup for portraiture, except that the 127mm would capture a little more than head+shoulders, considering you can't focus closer than 1.5 meters on this. Maybe a better alternative would be a Mamiya RZ with a polaroid back?
* I thought of using a Linhof 4x5 with a polaroid back, but shouldn't I just go 8x10 instead, if so, which camera could handle impossible 8x10 with acceptable results?