How hard would it be to get a low-mileage 2001/2 540i to ~350 hp? Super vague question, but that’s kind of the point.

Considering purchase price + first year of ownership, am I better off upgrading a 540 (which can be had with 75k miles for $4,000) or just buying an M5 (99% > $10k, regardless of mileage).

Chances are, my heart will be broken either way, but I’ve been on CL for 2 hours really letting my dreams build up so I thought I’d ask.