Upgraded suspension is great... except that the ride quality gets hurt badly (usually). Get some dampening-adjustable upgraded coilovers. This way, you can adjust how hard the shocks are (I'm sure you'd it want a bit softer for everyday driving, then stiffer during auto-x runs). Not sure how much you'd want to spend on upgrades though.
Don't get ECU software as it'll be a waste of $$$ and will barely gain power at all. Definitely consider at least a drop-in high flow filter for maybe another few ponies afterwards. Consider an exhaust upgrade, mainly the rear section, as the stock one is VERY heavy. Exhausts from many companies can shave off often up to 50 pounds just with an aftermarket rear section, while freeing up a few more HP too. The closer you move towards the engine though, the more expensive things will get exhaust-wise. The rear section exhausts aren't exactly cheap either, but they have two good sides to upgrading it.
Here's Turner's dyno of their modded 330. Quite a lot of money was put into the car (talking probably over $10k if you're having it installed too; not worth tinkering too much with power adders)
Make sure if you want to do auto-x events, that the other suspension components can withstand the stress. Definitely consider upgraded front control arm bushings (stay away from hard materials such as metal or delrin, as they aren't ideal for the street- too aggressive for driving on bumpy roads). Just get a set from Bimmerworld (Powerflex) or AKG Motorsport. It's one of the best things you can do for under $100 to improve handling.
Since your car does not have a limited-slip differential, I highly recommend saving for one if you really want your car to perform well. This will definitely help you in the autocross, but it usually costs a lot. You can search around for used ones, but there's always questions about the true condition of it, and then if you're not that mechanically inclined, you have to get a shop to install it which can also take quite a few good hours.