Until I got my CR500 (a week ago) I was riding my 1994 CR250 on 30-35mi trail rides every weekend. Yes, they can be reliable.
BTW my CR250 (power-wise) will smoke any 250 that is new in a showroom. Here's a link to him. (I'm not suggesting that you buy it, for reference only)
http://www.arizonatrailtalk.com/cgi/yabb/YaBB.pl?board=Classifieds_;action=display;num=1066668145
Reliability is 100% subject to the maintenance and repairs that the machine has received.
Generally for a given dollar amount (say $2000) you can get a bike in a certain range of years (say 1992-1998). The newer ones in your price range will have been used harder, the older ones... well that's a crapshoot.
Everyone is going to sell their merchandise for as much money as they can get, unless they have time constraints.
Here's my used bike buying advice.
Be in the car and on the road when you call the owner. Give him as little time as possible to prep/rig the bike.
If the motor is warm or you can't see the bike cold start don't buy it.
Don't buy a dirty bike. Dirt and mud can cover cracked cases & frames etc. If he doesnt' care enough to clean it, how'd he maintain it.
If ANY of the fluids are low, don't buy it. Even if the seller offers to quickly fill it. This even applies to the gas tank-But I'll let the owner fill it while I'm there, or I'll pay to fill the fuel tank.
If he makes excuses when you ask questions, he's a lair. When you say, "How did this break?" He needs to give you a prompt answer and take credit for it. If he says, "Oh Gee, I think I got it that way." Bombard the owner with questions. Even dumb ones. "How'd it get that scratrch." See how he answers.
Get the title. Verify when the title was issued. If he hasn't owned it long then he may be trying to unload it. Generally I wan't to see a minimum of a year.
Inspect tires, sprockets, grips, handlebars, etc. See how recently he has replaced them. If they are trashed, then he probably hasn't JUST put in a "NEW TOP END".
Check out the sellers cars, wife, and dog. If they are clean and well groomed than you can assume he takes good care of his bike too.
If he really, truly has to consider the offer you make then you are doing good. See if you can sense he's dissappointed about getting less $$$ than he wanted to.
The seller should be proud of his possessions.
There are THOUSANDS of used bikes in your area. Looking at them is half the fun of buying.
Don't buy the first one you look at.---Only once have I POSSIBLY regretted this rule.
Have fun and be careful.