I had a dr650 with a pumper carb and upgraded suspension, and loved it.
It was loads of fun right up to the point of the helicopter ride.
It was not a bad fall, but the weight and height of the bike was too much for my old bones.
Something to keep in mind when playing in the dirt, the more weight you have, and the higher the bike is, makes for a worse crash.
After the dr, I got a tw200, at least 50 pounds lighter, lower, and big fat tires, great for the sand, and I went places I never would have gone with a dr650. And it was easy.
I hear very good things about the wr250, but I think a klx with a pumper carb and maybe a big bore kit would be the way I would go.
If you are going to do things to a bike, a carb is a good way to go, and a well set up pumper works as well as FI if you are at a stable altitude.
Lean set vacuum carbs just suck, and really dull the performance of any bike that has one.
I find it fun to push a bike to my and its limits in the dirt or on the street, and find a smaller bike fun on the street, where you can pin the throttle, work the gears, tuck in and simulate a road race, without getting tickets.
The dr650 was great fun, but with the ability to get over 100 mph very quickly, the throttle wheelies, tickets were just a matter of time...
I did not do a lot of street on the dr, I hate the tire wear, wanting good sharp knobs for the dirt/sand/mud, but it was fun on the street.
In the dirt, I want light weight above everything else.
2nd comes reliability because I ride alone and hate pushing a bike in sand.
3rd comes power and throttle response.
4th comes suspension.
A friend has a couple of xt225's and they were a lot of fun in the rough stuff, very easy to ride over some very nasty stuff.
The dirt is very close to my house, so my need to do the street was nil, and a dr350 or some 250 would have been a lot better then a dr650, but other people need to travel to get someplace good to ride, and there a 650 is likely a nicer choice, if their bones are strong....
Anyone who rides a 650 in the dirt needs very good medical insurance...