What about a 350Z or G35? About as fast as an MS3 stock, but with RWD and a decent amount of upgrade options. If it's speed your looking for, I would choose an '00-02 TransAm with RamAir or even a Pontiac GTO over the ugly magnum.
Anything with AWD and a turbo is going to have an easy upgrade path, but Subies usually need the tranny to be built up for even modest power gains.
Personally, I'm quite partial to the 90's Twin Turbo 6-Cylinders. Supras and 300ZXs are both hard to find and expensive, but the market for 3000GTs is fairly decent sized. I love my 3000GT VR-4, its the only of these twin turbo cars with AWD. BPU upgrades in total costing less than $1000 easily let a car like this gain an extra 100HP, and many turbo upgrade paths are available.. The engine internals can take up to ~600+ HP at the wheels, and the transmission is fine other than the necessary clutch upgrade. Only downside with one of these cars is that your going to be paying a lot for regular maintenance, and small things tend to break somewhat often. But it's worth the price of admission, and a VR-4 makes a great project car.