a mild steel exhaust will last longer then 5 years, pfft as if it'll die on you, a stainless steel is made to last forever, why fork it out? i have had friends who have had mild steel exhausts on their car for over 5 years and nothing wrong etc.
Dont spend over $350, just shop around, get out the yellow pages, and start hunting, thats how i originally did it :)
With modifying your wagon and putting gas conversion on, there is no point of putting a Chip because the computer will not be used by an aftermarket lpg kit (normal lpg kit would cost roughly $1800..)
You can get a high performance lpg kit, and all most normal lpg kits including mine has a small computer anyhow inside the engine bay.
You can modify your engine by getting a cam done, then high performance lpg, extractors, so on so on, even maybe get the diff ratio changed for more acceleration :)