Seeing that nobody has responded yet i thought i would give a few tips. The things i will post below are not to persuade you in a certain direction, but a few things to keep in mind. I have had my 2001 NB for a few years now and have learned a few things along the way. Now I will pass them on to you.
1. Is your husband planning on doing the maintenance on this car himself? (don't have to answer just keep in mind). The beetle has the least amount of room to work on the motor, so depending on what is being done sometimes either more parts will have to be moved to get the job done which takes more time or you send it in to a mechanic.
2. If there is maintenance that needs to be done, and you don't want to do it yourself do you know a mechanic that will work on the car? Is he one you trust? (if you don't know one check the trusted TDI mechanics by state page)
3. As you said it can get expensive to have work done on the TDI's, but it will always depend on what is being fixed. These are foreign cars so the parts can cost more than U.S. cars.
4. A few things to think about for winter. i live in MN so it does get cold, and starting can become a issue sometimes. in other words you will want to make sure you have a good battery, starter, and glow plugs. Maybe heated seats
5. If the car has been taken are of well it should last a long time. At least 400k seems pretty common, but i have heard of people in the 500k-600k. there will be work that needs to be done from time to time, but hey if a gasser could last as long it would as well. Hopefully someone else in the high mileage area will have some better insight as my car is at 220k right now so i am not quite there.
If i think of more i will add to my list. I know you said your husband knows a lot about these cars, but i just wanted to point some things out that may not have been thought of. These cars are great the ALH is known to be one of the best TDI motors that VW made, so i am not trying to make it sound like they have a lot of issues. I love my diesel and wouldn't trade it for anything other.
Hope this helps,
GlowBugTDI