Not being funny.. But I'd want to know.
Going to a dealer doesn't mean you're getting a decent car. Sure, you might stand a better chance of getting your money back but then you're back to square one. Again.
It does feel like you're searching for a unicorn in a field of horses. If nothing else, this purchase has shown you that mileage, history, dealer (or not) is absolutely no guarantee that you'll get a good specimen that will give you minimal hassle.
Who's to say the next one won't have a similar issue (if diseasel) or the petrol variant you go for doesn't have another undiagnosed problem that makes it go pop within a few weeks?
P38 ownership really isn't about ignorance, you have to go in balls deep and know these arseholes inside out. Once you do, spotting the problems is a lot easier and therefore, the remedies are easier.
I still learn things when I start fiddling with it be it asking questions on here or from actually doing the spannering. Can't help but think while you have a decent budget to spend, you're still going to find disappointment somewhere along the line that could have been avoided if, perhaps, you went for something that at first appeared 'less' desirable.