Heya mate, I can tell you from personal experience, that price is a pretty fair one.
As for an RB26 into an S12, yes it's been done before. First was one in Finland several years ago. I completed the second one just last year and late last year a third one was done in Greece. That's all that are known to have a 26 swap.
For information about RB swaps (20-26) into an S12:
http://s12silvia.com/community/index...lly-addressed/
If you you want more pictures and have any questions I'll do what I can to help you out.