The spring inside the relay used to make a standard VW hard start relay is
rated at 100,000+ cycles at full amperage (as is the case with most high
quality relays).  It doesn't matter what type of load it is switching
(headlights, starter, horn, fan, radio etc), as long as the load is below
the rated amperage capacity for the relay.

The classic Bosch WR-1 12V 30A hard start relay has been used since the 50's
to make starting VW's possible when hot start problems arise.  It is sold by
pretty much every VW parts supplier in the world:

http://www.busdepot.com/wr1
http://www.airheadparts.com/vintage-vw-parts/starter-system-parts/hard-start-relay-kit-12v-bosch-wr1
http://vwparts.aircooled.net/Bosch-WR-1-Hard-Start-Relay-Kit-12V-p/wr-1.htm

In my 18 years of VW ownership I have used a standard hard start relay
(modeled after the Bosch WR-1) on every bus and van I have owned.  I use
relays primarily to protect my ignition switch (which probably explains why
I have never had to replace one),  The fact that the relay supplies
increased voltage to the starter is secondary to me.  I have never had a
failure of any sort with one of my relays and have only heard of one
reported instance where a hard start relay failed.

In short, you can purposely over-engineer anything, but you are not going to
get a better end result than you would with a properly sized, fuse protected
relay like the common hard start relays that VW's of all shapes and sizes
have been running for decades.

Jay Brown
'85 Zetec Westfalia
https://sites.google.com/site/vanagonheadlightrelays/