Two really good options here. The first is a '93/'94 325i. Has Vanos and replacing the exhaust, differential and chip can slice almost a second off ots 1 to 60 mph time which makes it a very quick car getting really close to a 3 litre M3's times at a far lower cost.
Ditto for the 328i which has a lot more torque and with a chip and exhaust is a far better option than an M3, will cost far less to keep and run and be just as quick.
All depends upon how much you want to spend. I would go for the 325i as the entry cost is less, I can spend more money on getting a really prsitine example with low mileage and finally I gradually upgrade the car to almost M3 spec without spending a fortune!
Hope this helps,
Geoff
Sydney, Australia
'94 525iA