There's quite a few to choose from, but consideration for road use would probably mostly be about top speed (i.e. you don't want to be doing much motorway work on a DT50!). But the off road aspect depends on what sort of off roading you want to do... green laning, motoX, trails... what do you think you could manage to drag out of a muddy hole?
I think a bike made as a lightweight supermoto would be a good bet, and my vote would go for a WR450. KTM450 is nice too, and there's a beautiful
CRF on ebay at the mo.
A spare set of wheels for mx tyres and you're larfing!