Hey OP I don't want to rain on your parade but that bike is overpriced, are you joking it's 5 years old (and ran hard in those years I bet) and they want $4,600 for it? Brand new is $6,000, or $1,400 more. I would not pay more than $3,200 for it.
3200$ lol. bikes looks good and 4000$ sounds about right. For obvious reason these bikes keep their value unless trashed. Hard to find all unmolested and ready for you to mod away. If it was maintained properly frequent oil changes etc seems like a good deal. Get him to write the bill of sale to like 2000$ save some money on the tax. Bike is bullet proof if maintained. Looks like new tire, avons...also unsure of the 70MPG thing... I get 40 but I ride HARD!
Few checks:
remove the sprocket cover and see if the C/S nut is loose
Check the sprockets for wear
chain lubed up
take plastics off look at all electrical you can see...no chopped electrical taped up wires, should be nice and neat from factory.
3x3 mod? if so needs jetting
Start it cold, choke for 25 seconds or so less, listen to it. It's a noisy motor but if it's really noisy could need a cam chain
throttle response should be pretty good...a wee hesitant with out jetting.
Ride it. go through all the gears.
check air filter....if super dirty it probably has been neglected...no biggie if you buy clean it!
ask about loctite fixes, if unsure of what they are get on Thumpertalk.com DRZ section.
I would only be worried if it was not maintained properly....most say oil change 1500-2000kms with good oil of your choice, valves should probably be checked but it's easy and takes 1/2 hour, 15 just to take the tank and head off. Good luck and dont be afraid of the Km's...many 50,000km DRz out there.
D
ps be sure to check the drain plug. over-tightening will crack the case...new crush washers everytime time!