At around 15,000 miles, its time for a cam chain, so, theres time & $$ in it. If thats done, then great, but, I wouldnt bet on it.
Or at least a Verslagen mod.

PIcs would be good, Tires are costly & having them mounted is nothing like a car tire. Its $30.00 for one around here. So, if tires arent in tip top shape, look to spending $150.00 or so to replace & mount a rear with a good quality tire.,& if he says, "Theres only 2,500 miles on that rear tire",, he meant, its 50% shot.


IDK what average prices are over there for a 26 year old bike, seems to me that would be around $1,200, & that would be about 700 too much.

At 2.5 they really haven't had the opportunity to screw things up, so, in my mind its more likely to be in a shape that in the long run, is cheaper.....
You might want to sniff the set too, just in case. It can reveal a lot about the person who rode it, just ask my dog.

Go for the newer model   for all the reasons stated   and for the ones you've already considered   you know that's what you want - it's just about parting with a little more green  I thought about 2 bikes that were for sale close to me a couple of months ago   but when my hometown dealership put one(actually two) up for sale : brand new 2011 models on clearance for $3,999 in August 2012   I jumped on it    

13,000 fewer miles for 1,150 more.  If it were me, I'd be a buyer of the newer, low mileage S40.  With any luck, other than minor oil/filter changes and new tires, the newer S40 should be relatively trouble and expense free.  At least that's the theory, your mileage could vary.
Best of luck with the one you choose.