I got a 2008 Honda CRF250R with SM wheels.
Its the best of both worlds for mucking about. Reliablity of a 4 stroke, but still performs ok and is enough to have some fun.
What exacttly will you be using it for?
If you are set on a SM/MX style bike, i would say a 250 minimum. Maybe looking at a KTM Duke or something like that if its for the road/messing around.
If your actually going to be going offroad, then maybe look into a 2 stroke.
My bike on the right