What is the expected mailage out of a 98 GSX engine?
What would be signs that an overhaul is needed?
What part fails first?

Thanks.

The only question here I can really answer is the gas milage one. Roughly 20-26. That is of course if you can keep your foot out of it, I know I couldn't.

Just lookout for burnt o2 sensors, those will impede you gas mialege by alot.

A compression test will tell you if an overhaul is needed