Boy, I hesitate to weigh in, so much to say, so little time. The key is maintenance. Rebuild & replace everything if it hasn't already been done. My history with Vanagons (3 of them, 2 water-cooled 1.9L, one air-cooled 2.0) finds that, averaged over the years, one may expect to invest about $2000/year in maintenance, unless you do your own work. Known issues such as Vanagon syndrome (there is a fix) and blown head gaskets are familiar. Then there are the weird things, like a short on a wire causing the oil light to come on, floppy mirror syndrome, temp sensors failing, etc. It must become a love affair or the relationship will not survive.
I agree with what the others have said - except maybe I've been a bit more lucky. I have yet to have any major head gasket issues - but it is common. I've also never had a water pump issue.
The cooling system in general is a pain in the but. Also, Vanagon syndrome can be a bugger to trace down (because it can have multiple causes) - My advice would be to make sure that you a) Get a Bentley manual and b) sign up for the Vanagon e-mail list at vanagon.com
Now having said that I will say that similar to what I've been told about older buses - Vanagon's are remarkably easy to work on - I had zero mechanical experience before owning a Vanagon and have found I've been able to do most jobs myself - with help and advice from others. Right up to a whole engie removal and replacement.
They are great wonderful beasts - but can be frustrating at times. Keep up the maintenance, replace those fuel lines, and replace anything that might be suspect (within reason of course).