if u want power go the exhaust, that will net u a bit.... if you want to push over 190kw... get a fuel pump, injectors.. but most of all get a new ecu, as the stock one is rendered useless at anything past 10psi= R&R.
at this point u will need a clutch..250+hp, get a basic heavy duty one. anything over 300kw = carbon twin plate clutch.
but your on a budget so i would go the =TURBO back exhaust as the exhaust is only as bit as its smallest diameter piping, so a catback is useless..a bosch 040... then save for coilovers and ecu. that should get u to 250hp region.\
better having coilovers as a safety issue. the suspension is meant to be replaced every 10yrs, so its not a speed component more of a stability/safety issue