For $18k, (IMO, and for what it's worth) I'd get a 2001-2002 Audi S4.
A great all-in-one car with potential, good looks, , comfort, reliability, and the best AWD system offered by any manufacturer.
Although not cheap or simple to modify (due to the lack of space in the engine compartment), the engine is good for 500+whp with bolt-ons, and the rest of the driveline has been proven to hold over 800whp.