Hi Dura
Welcome to the forum.
If you can get the maintance records you will have a better idea.
Others will chime in that have high mile rigs,but from what i have read and heard on here i wouldn't look at those being high miles,more like mid life for a dmax,and i'm assuming an allison trans.Look at the front end for possibale new parts or evidence of tire wear on the inside of the tires,one of the ambulances in town here has needed front end work,and its about the same yr.It probly wouln't be your daily driver so in a few yrs the milage wouln't look so bad.
Sounds like a clean rig,look it over and find all the little things it needs and point them out and knock the price down,or have them fixed.
Good Luck
Thomas
90 Chev 3500 c/c 4x4,6.2na,400 auto,4:10 gears.DSG Timing gears,main girdle, isspro tach, pyro,boost,oil and trany temp.Dual Tstats, High volume peninsular pump,on shelf, Custom turbo and intercooler 85%complete. Change of plans for the dually, it's going to get a Cummins. Both trucks are Blue 90 4x4 crews