BIG difference between CRF250R and CRF250L. The R is a full RACE bike, the L is a mild dual sport bike, much more reliable on a longer trip.

Also, same sort of difference between the Yam 250R and F. The F is a race bike, the R a mild dual sport bike. The R is preferable for travel on mild Pistes and such. A great bike but EXPENSIVE.

Either Honda or Yam road versions would be good. But if you're new to dirt bikes you need to find a riding partner. Don't go there alone. You WILL fall as a new rider. Most times you will be unhurt ... but if you have bad luck you could get into trouble, especially in bad weather. Buddy System is best or join a tour.

A tour will take you places you could never find on your own ... but you must PAY for that privilege.

Perhaps buy in EU where CRF250L is sold, buy a nice USED one?
Must be a way to register bike in your name and avoid import tax? Buy in France, keep in French plate/paper?

I think it's a good idea to put bike on trailer. We do similar thing sometimes when riding Baja, Mexico. We drive and trailer bikes to Mexico. Then leave truck, go riding on full dirt bike (not legal to ride in USA roads).

We carry small luggage, stay only in Hotels, eat in restaurants. A real vacation.

(sometimes even a "chase" truck to carry luggage and other luxuries )