There looks to be a shock absorber trailing the rear leaf spring in one of the photos. Possibly to help mitigate rear axle wrap with all that torque? Might ask him about that.
I'd check out and ask about everything you can, make a list of questions. Ask him what is wrong with the truck (there is something wrong with everything) and what else he would do to it if he were keeping it to make it the way it needs to be in his eyes. He built the truck, so pick his brain.
Ask him if he ever has to add any oil to the engine between oil changes. It's a low mile engine, but don't assume anything.
Ask about the egt temps as mentioned. And ask about how hot it runs, if it overheats ever, if it has ever overheated.
Inspect all the universals, front axle universals, front and rear drive shaft universals. Ask him if he has had any problems with u-joint failures, perhaps due to new angles and torque. Check the king pins, those are a pain to work on or have done.
Ask him when the transmission was serviced last and how many miles are on it, that's a pricey piece of equipment. Might take it in and have it serviced before you buy it, see if there is any metal in the pan, make sure the fluid is the appropriate color, see if things are way out of adjustment. If you have to take the truck in to a shop someday to get a new transmission it can run you $4000.
If I had this truck in my possession, I'd be turning things back down more towards stock configuration. But that's just me, I like things reliable. When things start getting tweaked and messed with, it cuts down on the life of the engine and the reliability IMO. The earlier 12v's were engineering perfection. They literally outlast whatever they are installed in and the people that own them. Can be unmercifully flogged, and they will not die. They just keep going and going. It's a beautiful thing.
All this in an engine that will burn just about anything with a carbon atom attached to it. Vegetable oil, waste motor oil, hydraulic fluid, BIO diesel, two stroke oil ... there are people that run these things and never pull into a gas station to get fuel.
And ... the things will move 5 tons of weight at 70 mph and still pull down 17.5 mpg.
I have a 93' 12v with 300 thousand miles on it. Starts every single time and never falters. I never have to add any oil between changes and the oil level does not change. There is no blow by, the thing purrs like a kitten. The truck is falling apart around the engine, lol. So after tie rods, drag link, brakes, cylinders, front and rear drive shaft rebuilds, carrier bearing, steering shaft, etc.... the truck is finally worthy of the engine. "Just" have all the spring bushings to do and she'll be as sweet as honey. Real glad the king pins are good.
Off the soap box, I just love 12 valves is all. Overall, that is one sweet rig you are interested in buying. I imagine it would be a fantastic vehicle that wouldn't give you much problems.
