If you don't know the mileage, there are several things to examine that are at least indicative of the service on the truck. Look at the spring on the top of the injection pump that allows the throttle to be pressed pass the internal movement of the throttle. The spring wears at the contact point of the pin on the lower lever. The amount of wear will give some indication of time on the engine. My 90k truck shows little wear. My 275k truck shows about 70% worn through. My 385k truck has worn completely through and will require that spring to be changed in the very near future.
Look at the crack in the chrome in the front (right at the safety latch on the hood). The length of that crack is an indicator of service on the vehicle. Probably all of the things like pedal pad, wear on the steering wheel, seat condition and that sort of thing would have been changed in the conversion. The four door cab would be no newer than an 85.
As to the price, if it is nice and the conversion was done well, it would surely be in the range of worth the asking price. I am in the begining stages of doing a fourdoor. I think that I will surely have 10K$ by the time it is complete. Most people have a posting price and a selling price but I have seen mostly firm no negotiation prices on the ctds. It never hurts to ask.
93ctd, automatic o/d , 3. 50 rear, running boards, 90k miles, box stock otherwise. 93 ctd, getrag ,3. 50 rear, airbags mounted on axle, 387k miles, box stock otherwise 92ctd, automatic o/d, 350 rear, airbags on spring ends,275k miles, box stock otherwise. Two 85 four door models for parts