There are a lot of things to be careful about when purchasing a Vanagon. If you haven't noticed that in the classifieds, a Vanagon is usually more expensive than other VWs of the same year. This was also true when they were new. Take example, an '85 Vanagon GL Wolfsburg cost in the area of $18,000 when new. That price is comparable to Cadillac's and other high-end vehicles of that time. The price is worth it though.
Keep in mind that all Vanagons shipped to North America were made in Germany. Most were made in the same factory (Hannover) as the earlier buses from the 60's and 70's. The build quality of the Vanagon has always been the best VW offered. And in some cases VW went all-out and lost profit just to make something different. In other words, VW made the Syncro Vanagon. This all wheel drive wonder was produced between 1986 and 1992. VW officials didn't like the Syncro due to the high cost of shipping the van to two different factories for assembly.
Finding the right Vanagon can be very hard. People don't give up these rides until they let them down. So most used Vanagons have problems. The smartest thing to do is stay away from the high mileage vehicles. Anything over 75,000 miles is in need of some repair. The air cooled Vanagons (80-83) have a 2.0 liter air cooled motor with fuel injection. It's a very reliable motor. In some cases lasting more than 100k miles. Some have A/C and automatic transmissions. Most have the 4-speed manual.The diesel Vanagons (82-84) have the same motor as the beloved Rabbit diesel. These motors are very reliable too, sometimes lasting 200k miles.
All diesels come with the 5-speed manual. This model has proved to be the best for engine conversions. Some people convert the motor to the gasoline version. Which makes a pretty reliable vehicle. Then there are the Waterboxers. Made from 83 till 92. These are the ones you have to be careful about. The motors on these vehicles aren't tolerant to abuse. $2,000 or more is a typical engine rebuild.
The biggest thing to watch for is the coolant. People always use the wrong coolant. Make for sure the coolant is Red or Blue. Not that neat looking neon green stuff. VW uses Autobahn phosphate free antifreeze. This coolant is expensive but designed so not to corrode the aluminum the motor is made of.
The Syncro Vanagon (86-92) is VW's only 4WD vehicle. VW assembled the van except for the undercarriage and then shipped it to another factory that installed the 4WD system. These vans are very capable off-roaders. The major costly thing to watch for is the Syncro drive train. The 5-speed transmission and the front differential is expensive to fix. Most wear out before 100k miles. The Vanagon Westfalia was made for all years and is basically the same as a normal Vanagon except that is has more rubber seals to go bad.
In general watch out for any leaks. If you see coolant on the ground (especially the green stuff) deduct $1500 from the price. If you see rust, deduct $1000 from the price. Most Vanagons are reasonably priced so you don't have to worry too much about being scalped. Another thing to be aware of are the correct tires for the van. The Vanagon uses a D or C load rated tire. These tires are very hard to find in the correct size. There are some special edition Vanagons out there that bring in the money. A 91 Vanagon Wolfsburg Carat brand new was $23,000. $30,000 used in perfect condition.