Welcome to SAU, and to the Stagea section.
First things first, where are you located? If you can give us an idea, then we can give you better location-based information in regards to who to use to buy and maintain your car.
Manual Stageas were built in lower numbers, so that's why they're harder to find, and (usually) cost more. Apart from the 260RS, there were no other manual s1 Stageas in the C34 range.
Nope, they're not all AWD, so if you wanted a RWD they are out there. But I think all the RWDs were autos, someone may correct me on this...
Engine availability question should be able to be answered in the Stagea Buyers' Guide thread. The one with the RB26DETT was the 260RS.
Yep there are a few conversions going around, but it would probably be cheaper just to buy one with the intended engine already in it.
Yes, but getting the kit to cover where the rails went isn't too easy, AFAIK.