If you need 24 volt parts for a 5.9 look at a clark/michigan 75 payloader, ours had a 5.9 that was all 24 volt. Personaly 12 volt for what we are doing is way better. What if you are on the road at midnight and need a head light in 24v? or you need a jump at a truck stop? I dont think you are ever going to start the thing at -55. A 5.9 ALWAYS starts good even on 12 volt, just buy GOOD batteries.
I paid 2,200 bucks for my motor and got everything from aftercooler to drive shaft. If you go looking for one expect to pay in he range of 2,500 to 4,000 for an industrial engine. DONT BUY A DODGE MOTOR unless is is VERY cheap. you will need to go find a bell housing and they are kind of in a whole world of their own.
while you are at it, put a double disc clutch in, it will be alsomst the same price, I have the NAPA number. The flywheel to get tied on is going to be your biggest part. I looked for a month and found ONE in a junk yard for $350 but a new one is $900-1,000. I need to take mine apart to get the number from it, might be a while.