One of the best older used cars for the money is an E39 M5 BMW (1998-2003).
First of all it's a saloon, so the insurance is more reasonable. Secondly, it has a 400 bhp V8, along with all of the handling you'd expect from an M series. The E39 is also exceptional even by M-Series standards. You'll get 22-25 MPG if you drive fairly conservatively, which is hard when a car is this ****ing awesome.
An E39 in decent shape with decent miles can be had for between 15-20K depending on where you live.