All of this assumes he is stuck on buying a used TDI Jetta, which is not what my first choice would be with $25k, but different strokes for different folks.
When I suggested that a gas Jetta might be more fitting for his lifestyle I got jumped on by a few people who refuse to acknowledge the pros and cons of diesel power. I have a 98 Beetle with damn near 300K miles...and an eGolf with 10K miles...and a few in between (traded up from a 2012 JSW to a 2015 GSW).
The fact of the matter is that people are repeating certain "factoids" without regard to context. A diesel *can* motor on for far longer than its gas version, but that's only under certain circumstances. There's nothing that would prevent a 1.8 Jetta from hitting 200K and that's not a particularly high mark to hit for a diesel anyway...
depending on how it's maintained and driven. If you put 5K miles on diesel by way of short hops through town that's going to wreak havoc on all the things that lend to diesel longevity. More to the point here, the city mannerisms of our cars are taking the hardest hits from the fixes.
You enjoy that punch off the line? That's gone according to widely reported experiences--mine included. Of course, you have a manual so maybe it's not as hard hit but you can get lots of pull from a gasser. If you want punch, and you have $25K, try a GTI. Better yet, grab an eGolf for about $5K out of pocket for a 3 year lease (accounting for the $2500 California EV rebate; varies by state) or buy one for a song. If your trips are under 100m round trip, you'd be able to renew a lease for similar pricing driving a <3 year car for anywhere from 10-15 years!
That's just one option. Like I wrote earlier, and some seem to have mis-read, I don't know your driving needs/wants. Saying, "if you bought it for X, it might not be as good a fit as Y" is not judgement or assuming. A statement of, "if you wear size 32 pants, you should buy 34 because these run small" is a conditional statement without any judgement or assumption about you or size 32 pants, in general. It's simply, "If X is true, then Y is..." but if X *isn't* true then it doesn't apply.
But realistically, why so hung up on this 200K point? You're driving 8K per year. Did you run the numbers? I did...it'll take you roughly 25 years to hit 200K. To be frank, unless you are being melodramatic, the amount of stress indicated in the thread title makes it seem like some people won't live long enough to worry about how long these TDIs will or won't hold up.