2003 cr250r

You will have to premix the gas.
As far as things to watch for, mostly they are the same as any bike.
Service the rear linkage on a regular basis.
make sure the powervalve cables are adjusted correctly.

~4 ounces of oil per gallon will be 32:1. I have a 2001 cr250 and the only thing I really ever have to keep an eye on is the spokes. I dont know if my bike is just weird or what, but the spokes always loosen up after a few rides.

Other things to look for:

-Worn chain and sprockets. Is the chain look like it's sagging way too far? Can you see if the owner has oiled it regularly? Or is it dry and/or rusty? Sprockets should be fairly clean, with the teeth being even on both sides. If they are overly hooked (like a shark's tooth), or have small burrs of metal on top of the teeth, it's a good sign that it they will all need replacement.

-Check the front forks down by where they connect to the wheel - if there is a ton of oil, or oil drips, it will need fork seals.

-Take a peek in the oil fill area. Warning signs here are a strong "burnt" smell, very very very black oil (this one is kinda hard to tell). Make sure to ask the owner how often he changed the oil, and how often other maintenance was done. Did he/she do the work him or herself, or have a shop do it?

- Kick the bike over...use common sense when listening to the engine. Feeling any "grinding" as you kick it over isn't a very good sign. Two strokes will sound rougher than a 4stroke, most definitely, but it should rev up pretty smoothly. A tuned 2-stroke doesn't usually idle for very long at all (if at all).

- Take it for a test ride if you can, make sure the brakes work well, both front and rear. Bounce on the suspension to be sure there are no odd catches, or that it's ridiculously soft. As you're riding straight, check down to see that your wheel is true, and not wobbling from side to side (bent rim).

- Hop off and do a walkaround again, check the frame for cracks, and for the bits you can't see, look at the plastics. Bent handlebars and excessive amounts of white stress marks or cracked plastics would generally indicate that the bike was crashed a lot, and possibly abused.

- Ask the owner stuff like the brand of oil he/she used, and what premix ratio. I normally go with 32:1, but I'd suggest going with the same setup that the previous owner did. When was the last top end rebuild done? Has the bottom end ever been redone? Was it raced?

Lastly...don't believe everything he/she may tell you. Let your eyes and ears answer most of the questions you ask.