hi, the otr give the engine better breathing and the exhaust has a slightly heavier rumble, get a 3" cat back system (not an attraction for the boy's in blue-- sorry black -- if you're in Victoria). most exhaust shops can fit them at a reasonable cost. OBD scanners help diagnose problems. there are a couple of excellent books on the topic, worthwhile reading. you can buy a HUD off eBay for between $AU45 and $AU75 I have 2, a W02 and a W03. (2 different cars WH2 statesman and a 1998 9-5 SAAB) suggest you do your due diligence on OBD and HUD devices. the HUD units display speed, revs, temp, battery voltage, distance to empty(DTE), instantaneous fuel consumption. the HUD's work on vehicles with an OBD@ connector(16pin) the wh2 has this connector. love that car. cheers.