Dont know a massive amount of mods to do to the fuel injected engines to get more power out of them (other than dropping in a 16v engine) so i'll leave that to the lads in the know.
Sorting out the brakes is essential though. mine would just about stop when it had a 1350 in it. braided pipes and a bracket to help stop the master cylinder flexing are good placed to start. Ive also heard the the brakes off the GTA are a fairly simple mod but I've never really looked into it.
Ive powerflexed the front of mine and it helped tighten things up considerably. Then wider wheels and lower it a touch and you should be good.
If you find the driving position hard to get used to a useful mod is to unbolt the seats and swap the spacers from the back to the front to give more support under the knees.
Things to look out for are few and far between. Dodgy second gear, rusty fuel tanks, perished fuel lines and rust in the spare wheel well. Oh and when servicing make sure you have an oil filter with one of the non return valves as it'll help prolong the life of the engine. Other than that pretty bomb proof
(Had lots of other things too as it was my track day car.)
Excuse my ignorance but are you talking about 105 series GTA brakes or far more recent 147/156 GTA brakes. What does the kit need to consist of, where can I get the stuff from, how much & is it a straight swop?? I have a 1500 8V 33 race car, running on 14 in wheels & the brakes are its weakest link.