I can't answer your first two questions because I don't tow anything, and probably never will.
3. The 260RS (both series 1 and 2) use R33 GTR running gear, so if you want to get a PowerFC for it, you need the one that works with an R32 or R33 (it's the same unit).
4. The oil gallery mod is pretty much the same for all RB26DETT's - if you're going to be using it at heavy loads and high revs for an extended period, it's well worth doing. Basically, they put a small restrictor in one of the oil galleries going to the head so the oil flow is slowed down. One of the most common reasons for engine failure in a GTR is oil starvation at the bottom end, so the crankshaft bearings aren't being lubricated properly because the oil has been diverted to the head. The restrictor reduces the risk of this happening.
5. The head on an RB26 is different to an RB25, so I doubt it would work (but I'm no expert here, so do some more research or wait for someone else to answer).
6. There are front mount kits available, but the standard intercooler in a GTR (and therefore a 260RS) is very efficient. If you end up getting one, take the intercooler off, give it a good clean, check it for damage, and if it's all good then just put it back on. There's a DIY thread for cleaning your intercooler, just do a search for it.
7. AFAIK, I've only ever heard of one 260RS with the factory sunrooves, and the authenticity of that car was a bit questionable...
For answers to the rest, just have a look in the Buyer's Guide and/or do some searching... Most, if not all, of your questions have been answered before.