Her are a few tips on buying a used bike.
1) Ask how and where the bike was maintained. On most bikes the owner can change the oil and filter with the OEM tool kit. Coolant is a bit trickier, and done wrong can cause big problems down the road. If the owner does everything themselves, do they use a shop manual, or do they "know what their doing" because they've been doing it for years. I do much of my own work, but I use a shop manual. If they use a shop, get the name and call them and ask about the bike and it's history.
2) Find a local shop in your area, and ask them about their experience with these bikes. Go in and talk with them a bit so they know your serious about riding, and want to have a good safe bike to learn on. If they know they'll get some business from you they will be MORE than happy to help. Ask if they will look at the bikes for you before you buy, to make sure nothing is wrong with them, and how much they might charge. $50 is a small price to pay to avoid a bad bike.
3) Ask each seller if they will bring the bike to your shop to have it looked over, if they say no, walk away. If they say yes, make the arrangements, see if someone from the shop can take it for a test ride for you (they will know what to look for, listen for, and feel for more than you). Have the cash in hand so the owner doesn't have a reason to say no. And agree before hand to buy the bike only if the shop says it's a good buy.
4) As an extension of number 2, Find a good local shop, either a dealer or independent, and form a relationship with them. Even if you plan on changing your own oil, they will probably doing the big stuff (tire changes, valve adjustments, etc.). They will help you pick out riding gear, clue you in on some good rides and rallies, and it's a great way to meet other riders in your area who can help you become a better rider your self.
I personally own a 1988 Kawasaki Eliminator 250, same basic engine as the Ninja 250, with 20,020 miles on it. And although Kawasaki says the service life of the engine is 30,000 miles, many riders have put 50,000 or more on with no major problems. The reason you don't see these bike for sale with high miles is because we hang on to them ' till the last piston stroke.
Good luck in your choice, ride safe, and keep the shiny side up.