tbh mate, you might be better off just concentrating on replacing consumables, and keeping it looking tidy, as any 'mods' you add probbaly wont have any noticable effects, you will be better off spending your time making sure all teh standard stuff is in working order and running efficiently.
clean it all down, gunk the engine bay, give it a scrub, clean it all off and then you'll know what condition everything is in... just get it looking smart so your proud of it!
pop some engine flush through it to de-carb it and then replace the oil and that might make the engine a little more efficient. also run a decent fuel addative through the system, will help clean up all the lines etc.
basically what im getting at, is give it a damn good service, especially if it hasnt had a good one for a while, and you should notice at the very elast the car will run alot smoother for it.
rob