Shazbot,
My RM-250's crank to rod bearing gave out about mid season last year. The bike was purchased used, so I had no idea of the engines condition. On top of that, I rebuilt the top end about 4 months before the bottom went. If I'm not mistaken, a 125 should have the bottom end done every1-2 years. and a 250 every 2-3. Frequency and riding styles greatly affect this too.
I am very mechanically inclined, but I didn't have the special tools required to do the job correctly. I sent my engine to Eric Gorr. He rebuilt the bottom, seals and bearings, and did a complete top end. I had it back in less than a month. The engine sounded tight and was much easier to start. My cost was in the $700 range.
I think thats cheap insurance for knowing your engine has been built correctly and you won't need to do a thing to it. Except ride that is.
Bottom line, I'd buy it and send the engine off to Eric Gorrs.
Hope this helps, Steve.