If someone said "doesn't run, its the cp4" that's a big red flag. Is it possible to negotiate a sales contract wherein you agree to a total price and subtract the cost of the repairs at an agreed upon shop? If you're looking at a cash purchase this might be a great option. You will get the truck at a reasonable price.
You would have to dig in for a couple hours to even verify the cp4 issue. You could end up finding more problems beyond that. What happens when the motor has a cylinder with low compression? In this scenario I would negotiate a price minus repairs with a floor price of $X. A new motor AND fuel system could eclipse the value of the truck so a floor would protect the seller and atleast give them scrap or part value in the deal. If they won't, let someone else gamble on that.
Cp4, rails, sensor, regulator, injectors, lines, tank flush, etc adds up quiiick just by itself.