I think there are some misconceived notions here. First, performance injectors won't do a thing for you without a good chip. If you go that route you get a set of rebuilt injectors with larger nozzles. Second, performance injectors are cool but not in a work truck. Injectors + heavy dump truck = lots of broken parts and short engine life. If you want this truck to be reliable get a good chip set up for heavy towing, gauges and a downpipe.
Can you fix just one injector? Yes and I would only fix the bad ones. It's very rare that all the injectors are bad. Good reman injectors from Alliant Power run $215-240 each plus a core. You can get new Alliant Power injectors for just under $300 each. Alliant Power is the only supplier of injectors for Ford and International powerstroke and T444E engines. Are you certain that the truck has a bad injector? A bad wiring harness can cause an injector(s) not to fire. It's kind of uncommon to have more than one bad injector. Has the dealer run a system scan on the truck? What was the cylinder contribution results?
Lastly, that truck is way overpriced. If the truck is in really extremely good condition aside from the injector issue and has the heavy GWVR, 15-15,500 would be fair. Which wheel base, trans, and gear ratio does the truck have?