seriously, this car sounds like a rip off mate, there are plenty of good rexy's/sti's around with good upgrades where you can actually see where the money has gone into, like brembos etc. Honestly, if 30 odd grand has been put into it over time, he must be including the purchase price in that from 15 years ago... The only person who would put an old Legacy motor into a GC8 is someone who is doing it on the cheap.

If you find a nice STI with a rebuilt motor with no receipts like you say, just take it to a reputable workshop and get it checked over.