you say its been well looked after and never boosted by more than 9psi and you trust him because he's been your mech for 5 years... so why has it had the gearbox replaced? Why has he even swapped engines?

You even said he's pumped close to 30k in it... I would be expecting a full fledged beast for that amount of money, not a fairly standard wrx. The whole thing sounds dodge to me.

Walk away, for starters its too much for a gc8, just because he's a long time mechanic doesnt mean he's going to do you any favours... especially asking for 15.

If you're looking at dropping v6 sti gear in further down the track, go buy a v6 sti now.