hi our first track car was completely standard bar being stripped out. we then fitted some koni suspension and bought a second hand set of semi slick tyres. they both made a massive difference to the driving, but bugger all difference to the amount o fun we had. you need to decide if you are building the car mainly for fun or mainly for speed.
the one thing which yill ruin your day apart from mechanical failure is break fade. we are still running standrd 8 valve brakes on the new car but with grooved disks (performance benefits probably negligable on a ar like this) standard pads(going for red stuff next time) dot 5.1 fluid and braided hoses. we have not had any brake fade with the new car at all. the standard brakes on the old car used to fade very quickly indeed.
new car has poly bushes which makes everything feel really tight.
if you have plenty of money to spend then go for the 16v brakes, good discs and pads, braided hoses and 5.1 fluid. drop the suspension 40 mm , uprate the anti roll bars, poly bush everything, upper and lower front strut braces, rear strut brace, roll cage, plastic windows, race tyres etc etc. or just uprate your brakes and spend the money on more track days!
as or first time out, airfield days are usually good as you tend to have more run off if things so wrong.
where are you located?