As someone who races (admittedly I spend most of the time at the back of the grid....) I'll add my bit.
At the end of the day, you can spend as much as you want, but you don't have to go mad. For those that have seen the Alfa racing, there are some very rapid 'production spec' cars, in fact some lap quicker than slightly modified cars. Prod spec cars run on standard cranks, conrods, pistons, cams, carbs/injection, standard brake calipers and discs, some headwork is permissable.
Ian is about there, weight and suspension are the key, you need to keep momentum going. Make it go around corners before you waste cash on the engine. Bear in mind you may have to drive the car to and from your track day. My prod spec Sud racer has 650lb springs, try going over pot hole with those!
For brakes, agree with Ian, get a set of Ferodo DS3000 pads for your standard calipers and discs initially, DS3000's are excellent for track days. If after that you want to go for faster times, then think about some engine mods, and perhaps some bigger brakes. I personally would go for a close ratio gearbox before both of the latter. Other than Snetterton you will never see 5th gear with a standard box. Tyres are probably more important than big brakes.
The argument about centres of gravity is a litte pointless at this moment, as until you are going for some kind of fully modified machine and you can drive half as good as a professional race driver, you are not gonna notice that much so long as the suspension is reasonably well set up.
Try Brunswick Motorsport, they run a turbo 155 (ex Tarquini), a Group N 156, and a prod spec 155 2.0 racer. They can supply calibrated shocks, springs, and will build you an engine or full car.
Peak Alfa run a slightly modified 155 V6.
Peter - If you go for a full monty track day car, you may as well be racing it mate.