"(he said that such an STC is a lot of FAA bunk and unnecessary)"
If that is his attitude, I'd walk from this one. What else has he done or not done?
STC = $1/hp or about $100 for a "legal" paper. Doesn't change the fact that the autofuel was safe to use without the STC or that It's any safer with paper in hand. So, does a law requiring you to have it, make an owner unsafe because he doesn't have it?
As far as the 172,, seems a bit pricey,