Trojan Motorsport in mitchell are pretty good for services and the like.
as for rego, if it's "un-modified" you don't need to go over the pits at dickson, I took mine to "Premier Automotive" in mitchell and they passed it straight up.
If they won't let ya go over with 18's, I'm sure there's someone with a set of 17's around here you could borrow... I've got the set that came with mine, BUT 2 of the tyres are a bit bald
With your boost guage, you said it moves 1/8th of the way to the top? This sounds very wrong. at full boost (7psi) it should sit on the second line down from the top, 3/4 of the way to the top...