that rebuild kit must include the crank, rod, and associated bearings, not just top end parts. It's about $200.00/year for normal 2 stroke top end maintenance (once on 250's and twice on 125's). This is based on about 6 months of riding. If you ride all year long, double that. For good performance, 125's require new tops about every 10-15 hours(15 hours wide open is a lot). 250's go about twice that amount or better.
Back to the "what bike" thing: If you will primarily trail ride at moderate speed (sounds probable, no?), then there are some bikes like Honda XR400's and the Suzuki DRZ400 that are good, solid trail bikes. They will have too-soft suspension for jumping big. Small jumps will be fine. Motocross bikes, like the YZ250f that you mention will be a lot stiffer ride and more stable at speed. They will also require a lot more engine maintenance as they are tuned for much more performance. They will also be more demanding to ride (they work better at speed). The aforementioned "trail" bikes will require almost no engine maintenance, other than oil and filter changes. They do not MX well, however, except in experienced, smooth hands. I get the feeling you want to trail ride, so I vote for a trail bike. The KDX220 or 200 is a good 2 stroke trail bike in your price range, and it is quite a bit lighter than the xr and drz. It also has just enough power to make life interesting.
voodoo dude, you're hijacking this thread!