If the car is not broke WHY FIX IT??? More then likely you will fix things and the things you don't will strand you!!!!
If the valves start burning oil, which they will with these cars, I would go ahead and fix that. But replace the intake? For what reason?
About the only tune up thing I see would be the Plugs and wires and perhaps the fan belt and tensioner. If you got money to burn, maybe do the stuff like the Starter/Alterator/Master Cyclinder these types of things, if orig on your car now, most likely, will be the first to go, not the INTAKE!?!?!? Sorry that one just makes me shake my head
If your car is not pinging or anything, you dont need to run the 93 gas. Time to time buy some gas additive if your jsut concerned about keeping the system clean.
AS far as the rear end fluid, I would go ahead and change them out. Probably do a radiator flush also. Basically, if you do not know the maint, history of the car I would go ahead and do the typical fluid changes and the such.
WELCOME to the site. I hope to see you post often.