The DR650 is no longer sold in Europe, you will find used models only that are getting long in the tooth and are expensive, they have not changed much since 1996, but if you can find a late 99 or 00 model in good condition they are a good bike to start with, but they need a lot of mods (suspension, seat, ergonomics, etc.) to make them decent.

If I was looking for a 600-650 In Europe id buy the XT600.

It doesn't matter as much as you think, even a fairly light DR650 loaded up with fuel, water and luggage will end up at ~ 200 kgs and will feel quite lardy.

A 400 loaded up wont feel that much different has a weaker subframe and will suffer from slower highway speeds

Ive done a lot of touring on 250's and while fun in the dirt, when you need to make miles it makes for long days with the throttle WFO, so you wont end up saving that much cash on fuel and getting anywhere takes a while - theres nothing wrong with this of course, if you have the time riding a 250 is great fun.

There's a reason that the 600-650 singles class is the most popular for overlanding, that's because they have the least number of compromises, they have enough power, they are not too heavy, they are generally simple to maintain and fix and reasonably cheap to buy.

The secret to overlanding on any bike is to pack light, pack low and central, this will keep it handling properly instead of feeling like trying to manoeuvre a whale.

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Cheers
Grif

'11 KTM 450 EXC
'09 Suzuki DR650
'00 Discovery Series 2 V8
'95 Defender 90 300 Tdi Overlander
http://gipperstravels.blogspot.ca