To be honest, my friend, I suspect that you have little recall.
The trouble with private car sales is that they're basically sold as seen.
I bought mine a little before yours (& paid a fair bit more); it has issues (had some, still having others, & new ones- I've been trying to address them..... slowly getting there). I've not phoned the vendor or been round. It sucks but the best I can hope for is getting stuff sorted as easily & cheaply as possible.
So far, it's had a major service (paid for, factored into my original estimate), new front differential, needs a new water pump, one of the rear brake cylinders is weeping & needs doing, clutch slave - similar. Heater mechanism needs some playing with, clutch pedal has developed a major creak, passenger window sticks down... etc etc. many,many more.... .. still things coming to light. I could kick myself.
I'm not happy. The seller may have been unaware of a a couple of issues but........ well not all of them......
In truth, the only options I think you have are to get shut 'as is' & lose a packet (I won't do that with mine - like you, I have ethics & won't pass it on without rectifying or admitting the faults), or bite the bullet & attempt to sort things (admittedly that's risky too, they can escalate).
If the guy will genuinely stump up 400 quid towards repairs, then take it & see what you can do with it. Yes, you'll have to pay more out still but possibly less than selling it as a project & buying another. You could still have a great truck, albeit for more than you'd planned to spend.
Mine's gonna have cost way over market value when I'm done, but I'd stand to lose more if I sold it & took a chance on another. I f....d up when I bought it, I'll have to deal with it now & learn a lesson.
I agree with you that 'going round with the lads' or whatever, won't solve anything. Some people could do with a smack but it's not for me to get involved. Understand the sentiments though......
However aggreived.. makes no odds now. It's about solving the problem. I'd take the guys 400 quid if possible, see what you might be able to do with it & then make your call.
Maybe not the most positive suggestion, but I'm not in a dissimilar situation and they're the reallistic options I've thought through.
Mine won't cost as much for one single job, but all combined? Well, probably pretty close.
Hope it all works out. The whole forum is behind you!
Ian