I think you are on the right track with 250's. I've ridden the CRF230 a lot and prepped one for off road for a friend ... so I know the bike well, have ridden it lots and know it's potential. The CRF230 is (I believe) one of the Brazilian made bikes. They are quite good, reliable. Don't shy away from the Brazilian bikes.
ALL GOOD.

The CRF230 will need to have the rear shock re-done and heavier springs up front. Carb will need a minor re-jet and less restrictive air filter. Do that and it is transformed to a really good trail bike. I'm 5'6" too, and have struggled with TALL dirt bikes my whole riding career. No fun.

If you have the budget you might also want to look at the new Honda CRF250L. This Thai made dual sport bike is really doing well and many are doing RTW on it.

Also look at both the Yam WR250R and KLX250S. The WR is Yam's top of the line in the 250 class, but expensive and rather complex. The KLX is cheap and cheerful but needs modifications to match the Honda (CRF250L).

None of the 250's will be "fast" on highway, save the WR250 Yamaha. But ALL will cruise OK at 50 to 55 MPH. But off road in mud and deep sand, the 250's will be a JOY compared to any 600cc class bike or even the older 250's like the XR250L, KLR250 or DR200. The 600's are way too tall, heavy for mud and sand. The old 250's are OK, but if you can
afford something better, do it. The CRF230 is an EXCELLENT choice, IMHO!