What I see is a guy who needs MX bike. Even though XRs and KDXs are great bikes, if you're going to ride on a track, they're not the bike to buy.
Now..... a 250 4 stroke is roughly equivalent to a 125 2 stroke. Most 37 yr. olds with off road experience wouldn't want a bike in that power range. So then you'll need to look at a 250 or larger 2 stroke or a 400 or larger 4 stroke.
Though I'm a fan of 2 strokes, I'd love to ride a CRF450R. Honda has been able to squeeze nearly as much power out of them as an open class 500 2 stroke, from what I've read here.
One last item. I rode dirtbikes from age 8 to age 18 and then quit riding. Got into street a few yrs ago at age 38 and back into dirt last fall.
What did I buy? A KX500. I don't ride MX, I trail ride mostly with my 6 yr old. But when I get a chance to get out with the guys, I have the most awesome dirtbike ever mass produced on planet earth. It's not hard to ride or start, like some will say. It idles nicely. And because there are very few parts to wear out and it doesn't rev high, it's a very low maintenance machine.
PLUS: They're very inexpensive if bought used.
Just my 0.02. YMMV. And all the other usual and customary disclosures.