What age is your car? if its pre 94 then you can fit a decat pipe without worrying about refitting it for the mot, and do a bfi on the intake with a k&n panel filter, that may make it a little bit more growly but not to noticable, fit a fuel cut defender and a manual boost controler and run it up to 15psi, that should give you about 300 odd bhp give or take a bit with no visable differences outside an just a little louder! will cost 250 ish,
it would also be an idea to change your fuel filter and run some fuel system cleaner through to help it run as it should.
Without doing the exhaust you wont make the most of any other mods, you could change the exhaust downpipe, but cant say what difference that'd make, and just depends how much you want to spend, you can get some jdm supratt 440 injectors, which are a direct fit, walbro 255lph fuel pump, emanage, cams, a front mounted intercooler would be a good addition but is very visable, its possible you may be able to get a uprated sidemounted intercooler! just depends how much you want to spend an what you ultimately want from it!