All I will say is once you have towed with a diesel you will never have another gas vehicle again if you tow allot. There is a simple reason why all those trucks you see hauling things are diesel. As far as that year its a good year. One HUGE problem I see with this truck and what he has done to it is the fact that he removed the fuel plate, though removing the fuel plate in itself is not bad which is what I have done to mine, the transmission is either shot already or going to be real soon. Anyone telling you otherwise has not done there research or had a truck like that with those mods. Good mods for power but its kinda putting the horse before the cart. First thing he should have done which you will need is a good set of gauges for EGT, Trans Temp, Fuel Pres, and Boost. Then you will want Exhaust and Intake and then the AFC mods. Governor springs, timing and other things can be done after that. As far as price, in my area that would be an OK price but not what I would spend for it. I paid 8k for mine, its a 96 5spd 4x4 club cab with all options and 217K miles on it when I bought it. My dad just bought his 01 Auto 4x4 club cab with all options including intake and exhaust with 157K miles on it in sweet shape for 9K. Bottom line shop for a truck, try to find a stock one is my best advice, and if you lucky like me you will find a one owner truck with all maintenance records. Took me almost 6 months to find one. Sorry for the long winded comment.
Also a quick comment. With that many miles plan on replacing stock parts very very soon such as ALL U-joints even in your front axles. Ball joints, Steering tie rods and trackbar, Steering gear box and Unit Bearings. About 1 1/2k worth of parts easily if you do the work yourself and dont buy cheap crappy parts but stock or better.