I doubt that he'd be racing 250 2-strokes with a 250 4-stroke - a 250 2-stroke is the same class as a 450 4-stroke.
On to your question of YZ125 vs YZ250F:
The YZ125 will be slightly lighter, and easier to start when hot as compared to the 250F. The 250F has a bit more power, especially down low, and will be easier to ride overall. The 250F will cost more to maintain as well, or repair if you blow it up. So, there's pros and cons to both. I'd say find someone with one to ride and see what you think, compare for yourself.
As for rebuilding the 2 stroke alot - hogwash. They don't require any more maintenance than a 4 stroke. Maybe if you were a pro, you'd need a new top end that frequently. Hope this helps