need some opinions, buying a used truck

[mtbxdrew]

Im looking to replace my 02 SD gas job that has been trouble free to 300k, but looking to do some towing and I know the gasser isnt up to the task, still runs great but its not gonna like hauling a heavy car trailer and jeep on it. which leads me to looking into some used diesels, not looking at anything besides ford.. Ive found a few decent trucks, turn key and a few fixer upers. I would really like to have a 6spd truck, always have driven manuals just my preference. I have found a 01 with a 7.3 6spd 250,000 miles, clean inside and out, has been messed with a little, 4inch exhaust, intake, gauges and a programmer I believe, asking 6k, but theres something going on with the motor or injectors.. current owner is claiming its just a injector, he recently had 2 replaced, doesnt make sense to only replace the 2, i know its a rather expensive repair to have them all done.. I've looked at the truck, runs decent, has a hard start and puffs a bit of white once it finally kicks over, Im thinking it could be glow plugs?, those were the symptoms for the 6.5td when i was a chevy tech...
Just looking for some opinions on the truck and anything else to look out for when looking at any of the older powerstrokes
thanks