The best bang for the buck will be the Mustang GT. While I really do prefer the German products, maintenance will be insane for the Audi's and BMW you listed. Especially if modified. The Mustang can be modified with either turbos or supercharger and will give you a seat of the pants ride. Check out www.thatracingchannel.com and search out the Mustang GT's. The only thing that outruns them are the heavily modified Nissan GTR's, but they are producing 1,200 hp.
My son attempted to modify a WRX - 3 times. It broke - 3 times. They cannot handle even 500 hp.
If you are really wanting German, do not discount a 2003 - 2006 Mercedes E55 AMG. Out of the box around 460hp (they are supercharged). You can switch out the pulley for $1,000 and have over 500hp of brute force that is as solid at 160mph as 60 mph. But limit that to a track, please. Tires and brakes are not cheap, however, and the vehicle has lots of computer components that are expensive. Just know what you are getting into with that rocket.
The cost of entry for the Mustang will be reasonable, same as maintenance.