Jeroen,
If you like the Yam 660 and plan LONG DISTANCE touring, might make sense to get another 650 class bike like you have at home.

We have several here for sale new and used, but as mentioned, you cannot buy the 660 here.

Of all the other 650 class bikes, my favorite by a LONG WAY is the Suzuki DR650:
Inexpensive to buy, tons of support, parts cheap and easy to find. Dead reliable, easy to work on yourself. Plenty of power, lighter than your 660 by 20 kg. or so. Smooth and can cruise at 75 MPH all day. And ... it's better off road by far than your
660 YAM.

You could also consider:
Honda XR650L
Kawi KLR650 (many for sale and inexpensive)
BMW F650 or G650 (various models) I DO NOT recommend the BMW's as they are generally less reliable, heavier and more expensive than ALL the Japanese 650's.

You could certainly go with a 250. This would be a good choice if you like to do lots of off road riding and will be FINE once you are out of USA and on smaller roads in Mexico, Cent. America and South America. Down there a 250 would be just fine. The pace is slower, roads are rough and small, so suits a 250 nicely. USA is HIGH SPEED in many places. (75 MPH cruising speed)

The Honda CRF250L is a good bike and quite inexpensive here. But the Yamaha
WR250R is the better bike, IMO. (I've ridden both) But it's also more money. $$$

I am buying a WR250 myself in near future.

I currently ride a Suzuki DR650 with 90K km on the clock ... NO problems.
Very tough bike, SO easy to maintain. Does nearly everything well.

For USA travel, a 650 is a better choice IMO. 250's are better on fuel
but fuel is CHEAP in USA so no worries. It's more $$ in S. America in places, but still not close to SUPER high prices of EU.

Best places to buy a nice used bike in USA are:
Los Angeles, San Francisco, Arizona (Phoenix, Tucson). Lots of used bikes for sale. Prices good, getting it into your name not a problem if you have a local address or buy from a dealer. Many travelers have done this before. Lots of reports here but somewhat hard to locate these reports.

You can buy a Chinese 250 in Mexico ... no idea about this, but know it can be done. I don't trust them ... yet.

Good luck!