The size and layout of the battery compartment is amongst the annoyances of owning a Vanagon. Especially the height limitation. The proper fitment for the Vanagon water cooled is the BCI Group 41, The air cooled used BCI Group 42. Both these sizes are becoming limited in choices and noe are available in AGM or other technologies.

For the Westphalia the fit is even worse as that box with sealed cover was added to isolate the battery from the "living" space. A bit surprising as most Audi's of the era with the battery under the rear seat had the vent-drain tube arrangement.

Anyway had the battery box been a bit taller than a BCI Group 48 battery would fit. These are current and common. These are also available in AGM and other special purpose versions

Recently I have installed a number of BCI Group 40R (posts reversed), AH capacity just slightly less but the fit is much improved. Especially on the 86 and later with that heavy starter cable. The NAPA ones also have a provision for the vent tube.

The 96R should also fit and while rare there are some available in AGM form, including Odyssey. ($$$)

There are a lot of factors including luck that effect the life and performance of a battery. I have seen many last for years and some fail in a year or 2. A variable is how well did they get formed, (initial charge at factory) and how they are charged and treated just after installation.  Abusing a new battery with a hard to start vehicle or non performing charging system will reduce its life and performance.

All that said a starting battery is a consumable and while we hate to spend $$$, the standard replacement cost is about the same as 2 or 3 tanks of fuel.

Decide on standard or if you want something better (AGM), (may not really be), price shop for best deal and availability and go for it.

Merry Christmas

Dennis