rowan,
ST246 is far from slow if driven correctly. I had the privledge of two of these . one was a bit slow - the second (N spec) was far from slow.
also with the EC5W - that is a V6 (6A13 I think) the amount of dicking around to make the thing fit and get the AYC behaving properly would be a nightmare.
I also get the idea that he wants an evo wagon. to me it would be easier to buy an evo 9 chop and build one. only issue I can see is the rear floor being different. (give me a current verion of CATS and I'll tell you.)
my 2c.