I started on an EX500. It was originally a 1987 design, which had some minor changes (fairings, brakes and a few other things) in '94, but is nearly identical year over year. Bulletproof motor, and a great starting bike.
Too many miles is bad, but too few is as well. I'd look for a machine that has at least 3000 km/year (personal preference). It really depends 1.) how much money do you want to spend and 2.) how handy are you with a wrench/willing to learn?
The best investment you will make (after safety gear) is the maintenance manual. It is very difficult to tell how someone has maintained a bike without taking it apart. Ask the seller questions like "after how many km do the spark plugs need to be changed", "what is the valve clearance inspection interval", and "what is the torque spec on the rear wheel nut" ? If they know the mechanical details, they are more likely to have done the work. If you just ask them "has X been done", they are more inclined to just say "yes". If they took it to a shop instead, ask them to show you proof.
Maintenance (or lack there of) is a bigger issue on entry level bikes because the owners are (I'm generalizing here) usually novice in their service skills (we were all there, so nothing wrong with it, its just a reality).
Before you buy, be realistic about the service costs of a motorcycle. They are expensive machines to own, neglecting maintenance (i.e. brakes/tires too expensive) can be fatal.
I always replace tires and clean/reasseble the brakes, brake fluid, oil, coolant, regardless of what the previous owner has claimed they have done. A bit anal, but how do you know for sure unless you do it yourself?