what sort of routine maintenance would a 0-3 year old 4-stroke ATV require? (ie. Honda 300EX or 400EX)...how much does it generally cost?

assuming no bad crashes or extra-hard use, what parts tend to wear down the fastest, requiring replacement (ie. clutch, chain, etc. ??)

anything to specifically watch out for when buying a used ATV?

 

4 stroke sport quads I'd assume normal stuff.

Chain, sprockets will wear the fastest I think. Check sparkplugs, bearring carrier (where the swing-arm holds the axel), actually any bearings period.

I think those are the vitals to check for. If you dig inside of it, check valve clearence, filter, carb,,, stuff like that.

 

Either of those Hondas will probably run forever, without any major problems. Routine maintenence: Clean air filter, change oil and filter, spark plug, valve adjustments. Tires, and occasionally a chain and sprockets. Anybody who can afford a bike, can easily afford to maintain it.