+1 to the above. The main downside of those Mk4 ALH cars at this point is simply that they are old and almost all have been owned for the purpose of getting lots of use and miles, so finding one that is not worn into the ground at this point is harder than it was 10 years ago. That alone is a reason to consider something newer but if you did, going much newer with a TDI gets into trouble areas the Mk4 cars never had. All around, the PD cars (2004-06) are good too though. The late era TDIs with common rail engines (2009-15) can do well for folks but are in a different universe of complexity and long term cost of operation. The B5 Passat TDIs with PD engines (2004-05) are good cars too but are also more complex than the Mk4 with rotary pump, and have some trouble spots that have to be addressed and can get expensive (BSM, transmissions primarily). The pre-Mk4 cars (Mk3 Jetta, B4 Passat) have their fans and advantages and are objectively good cars that can serve well for many miles, but are much more primitive vehicles for daily use and much harder to get parts for, plus the early 1Z/AHU engine has some failure points that were eliminated in the ALH. So that really leads you back to the Mk4 with ALH as the most solid option out there, even today when the newest of them is 20 plus years old.
All that said, a Mk4 ALH car can still be completely viable even well over 300k miles if given decent care. And keeping one going for the long haul is not hard as they are simple and easy to fix and still easy to get parts for. Still huge numbers of them on the road worldwide. As long as you start with a decent example that is not beat/crashed/rusted, keeping it on the road is a reasonable proposition. Just last week I picked up an 01 Jetta sedan with 385k on it that's a little tired but will clean up just fine, intended for a Smyth Ute project this winter.
The only caveat to add to Peter's good advice above IMHO is to stay away from automatic transmissions in the ALH cars. If you need an automatic then the consensus recommendation would probably swing more towards the 2004-2006 Mk4 with BEW engine and 09A transmission which was a massive improvement on the 01M auto that was offered in the ALH era. Also slight preference among most of us for the 2000-2003 models as the 1998 (NB) and 1999.5 Mk4 ALH models had certain small disadvantages. Not enough to make you avoid one if you found a nice early car for a good deal, but all else equal the 2000-up cars had updates that are preferable.
And since you mentioned "much more" power than stock AND indicated it is for your partner to drive on a daily commute to save miles on your truck (meaning you want it to be reliable), remember that you will be starting with a car that has already been on the road for decades and probably hundreds of thousands of miles. FORGET about modifications until you get the car to a very thorough "Stage 0", in other words plan on spending plenty of time on servicing and inspecting fundamental systems and catching up on deferred maintenance before you think about turning up the fuel and boost. Mods that put heavy pressure on things like cooling systems, old turbos, etc are not a recipe for reliability or saving money in the long run if they are done on an ancient heavily used car that needs attention to the basics first.