If you wanted to go that rount (4-stroke) then a CRF230, or a ttr-230, or a XR 250 will be fine for you...
But if you wanted to go 2-stroke, then any 125 2-stroke will be good..
From the bikes i listed they are both about the same in maintenence. And they are about the same "speed" the 2-stroke might be alittle faster, but you need to be on the gas, or in the higher RPM's, and the 4-stroke hase a lot of low end power, and you dont have to be in the high RPM's.
The 4-stroke will weight more than the 2-strokes.
Now if you wanted to go new age MX 4-strokes like the CRF250X, or a WR250F then that would be great, just a wee bit bigger than the 125 2-stroke, but it would require alot more maintenence. More often...
But you will be happy with anything listed above...