I passed up a ti, as we're trying to buy a house, but for 2k, and with the (mini) mighty m4x, I wish I hadn't. That car is an extension of the reason our engines are great. It's light, nimble, etc.
+1 for suspension. Bimmers are razors in the twistys and it's easy to forget how badly mileage beats on suspension. Refresh, rebuild, CHIP IT and enjoy. If you still need more - go boost or stroker. Properly done, 30-40% more power is easily achievable. Plenty of info here, and in a pdf from metric mechanics.
Metric Mechanics PDFIf you've got about $9000, they have exactly what you want...
Look at some of the turbo build threads. Lots of them use MS. It could be tuned for NA, but the motronic does a damn good job, and getting better than that would be tricky. Member Tglaze used to build plug-n-play megasquirt setups, but you need a lot of leg work to get results from any aftermarket ecu.
Good luck with your project, I hope you enjoy the road ahead, it's really rewarding. (especially if it has some curves)