A couple of things that I have found is that Exhaust - will give you noise and some extra power throughout ( maybe some drone around 2000rpm depending on the system ), Rockers - some mid-top end OOMPH ( the VY ECOTEC V6's aren't really happy HI revvers though ), LSD and Gears - off the line PEP and some squat, acceleration will be improved - probably the best bang for buck you could do, eg you could loose about a second off your quarter mile times with decent diff gears but maybe a few poofteenths off with all the others, a tuned PCM from MACE will make it more of a daily pleasure to drive with more responsive power throughout and quicker gear changes - do the other mods before this unless you have an OBD1 cable and the software to modify and re-flash the VY bin files, also the inlet and plenum insulators are not too bad for the price, better down low torque and response, but it all depends on what you are after a racer or something with looks, if you are after a racer why don't you keep it as a daily driver and put that coin towards something with potential or just chuck some boost down its throat, if you are after something that looks pretty just slam it on some mags and add some visual luxuries
IMO since you seem set on spending money is Finish the Exhaust, ADD some Rockers and fit an LSD with new gears to suit - ALL AT THE SAME TIME, that way it will give you the feeling of money well spent :beer chug: