Hi, What is the difference between the front sheet metal and how many panels need changing to turn a mk2 into a mk1?

Not possible. If you want a mk1 buy a mk1. If you want a mk2 buy a mk2.

I was thinking with the Mk2s being a lot newer and more of them finding one with a reasonable body may be easier.

I agree with Whooligan, get ur welding skills up to scratch....you'll be needing it ! With the amount of welding on my van I could've converted it into a London Bus as almost everything required rust repairs and/or replacing with new sheet panels !
Mine was a ultra rare old windowless RAA mk1 van, then it delivered ice to servo's etc which expains the hollow floors .
As for swapping the front to mk1 it can be done tho the main bit is where the wings weld to the inners which need reshaping. New slam panel as well as most the mk1 outer panels. Cutting and welding as you go..a lot of work and money..