Mine has 23K on the clock and it's a 2008, doesn't use a drop of oil and goes great - mos tof them were hard miles.
Pop over to
. : XT660.com The #1 xt660 resource : . | The #1 xt660X, xt660R and xt660Z Tenere resource, that's where you'll find a wholeraft of owners. One member has nearly 60K on the bike (a bike courier).
The bike doesn't really open up properly until around 10k anyway - but it's your money.
A few things to look out for;
Reg/Rec connector, some of these were shipped without being properly pushed together, and then missed at PDI. with 26K on the clock I'd wager that this one is fine.
Cush Rubbers, these are made from cheese and need replaced every two tyres, or every tyre change if you are unlucky. Worth checking the hub for movement as they retail at around £30 to replace. You can extend the life by packing out with inner tube, but it doens't last long.
Wiring loom wear, some early bikes had the ties overtightened and the loom couldn't move, as a result it wears through. A few people have seen this, but I'm the only one so far to suffer a failure from it.
Other than that the bikes are great value for money. Yes they will fur/rust in places if not properly winterised, no more or less than other bikes.
Aftermarket OE stuff, the crash bars have a habit of breaking at the front mount as do the Alloy bash plates. If the bike has them, check it - you could use it as leverage to push the price down. Other aftermarket stuff is available from Adventure-spec, Off-the-road.de and Touratech.
But the best thing is to pop over to XT660.com and aske the guys, you'll get lots of feedback.