Maybe i'm wrong but I dont think there was ever an rb25dett. I think they only made single turbo rb25s and twin rb26s. Maybee you should investigate these engines more.
Alot of people in south FL have done rb25's so there should be many shops that would do the work for you and wire it all up.... for a price. SRs have very similar wiring harnesses. They also use the same mounts. It's much easier to do on your own. but less hp. If i were you and had the money to do an s15 front and an rb25det swap, I would forget about it and just get an mk3 supra and put in a 1j.
if you make an s15 conversion and put an rb26 in it, you HAVE to paint it bright prism blue, put a body kit on it just like FF3 and a wing, a BIG one...that would be sooo sweet.
on a serious note... be original, put a g10-L in that bad boy, be smart...get good gas milage.(1.3 liters and 3 cylinders of geo metro muscle)