Engine studs corroded requiring engine rebuild/replacement.
Fuel tank corroded requiring replacement.
Rust spots under windshield.
Leaks, leaks, leaks - oil, coolant, brake fluid, power steering fluid... you name it.
Other general problems as you'd expect from a van this age.
These are probably the highest quality vans of this age you can get. My Caravelle drives like a Rolls Royce and is extremely versatile. Very few other vehicles would be equally at home on a campground, moving furniture around the city, and bussing 8 people to a footy game. Materials are generally high quality, and the build quality is good. They are made like tanks.
On the other hand, they are demanding and expensive to keep on the road. Despite the reputation, these vans are riddled with engineering faults that tend to rear their ugly heads at the most inconvenient times. The ones on the road right now will probably be on the road for another 20 years at least, with owners reluctantly reaching deep into their pockets to keep them running.
The main issue with the Wasserboxer engine is that if you put non-proprietary coolant into it - it will corrode the engine studs. Proprietary VW coolant costs almost $200 for 5L in Australia, so as you can imagine - no one except for pedantic VW enthusiasts ever bothers. Once plain water or (worse) normal green coolant enters the tank, the corrosion process begins and cannot be stopped. Once this has happened, there's really no point in flushing the cooling system because it will just start leaking. If left unrepaired, eventually oil will mix with the coolant via the corroded holes where the studs used to be - it's like a blown head gasket, but much, much worse. Fixing this requires a replacement engine (which chances are has the same problem, and you'd have no way of knowing without spending 6 hours taking it apart) or a very messy engine rebuild. Either way, your mechanic bill will definitely be in excess of $5000.
Another big problem is the mild steel fuel tank. In Australia we have standard fuel additives that are not very compatible with untreated steel. E10 compounds this problem. This combined with our hot, humid coastal climate means that you have about a maximum of about 15 years use before corrosion starts becoming a major problem... this is inevitable. A new replacement fuel tank will set you back about $800, plus labour, new hoses, new fuel pump, new filters, etc - you're looking at about $2000 including labour.
The stock exhaust is made from similar material as the fuel tank, and will also corrode in an irritating fashion, causing toxic carbon monoxide to enter the cabin due to the design of the undercarriage. This can be welded in some circumstances, but eventually gets so bad that it just has to be replaced. Fixing this with parts available in Australia is again very costly.
Mine does run a little rich, but even with the oxygen regulator gone - fuel consumption is outrageous. I get upwards of about 20L per 100km. That's around 300km on a 60L tank of premium on a good day. Appalling.
Do not have any illusions about this being an easy going hippie van that doesn't need much maintenance. It's actually a moody, temperamental beast that will take all your money, and occasionally pay you back with a memorable camping trip or a convenient furniture delivery.