Here is my  $O.02 based on owning 3 vanagon Westies and a Eurobago:

There were gradual improvements to the vanagon all along, but there is no
magical point at which they go from "bad" to "reliable".  The last really
big change was the switch to the 2.1 liter engine from the 1.9, which
happened in 1986.  If a vanagon Westy appeals to you, then by all means buy
the best one you can find or afford.   Cooling and head problems are
possible with any year, but the total package has at least average
reliability compared to vans in general.   And, of course, the camping
package offered on Westies is outstanding.  The 1991's bring a premium
because they are the newest, but there was no big technology change in
1991.

By the way, there was no 1992 vanagon.

The EV has a different set of problems from the vanagon, but has the
advantage that you can buy a newer model, more power, and better AC.  If
you like the EV, don't hesitate to buy one.  Although the EV Winnebagos are
often dissed on this list, I was very happy with mine.

Whatever you get, enjoy!