what coolant to use?

Since you already have a leak then there is no need to use anything but the cheapest because shortly you'll be putting on head gaskets and a water pump. Then for longest life get what ever version your VW dealer has of G11. Don't use dexcool!!! Head gaskets are a problem on vanagons in the US but not in Germany. Some say water, some gasoline. Who knows. My 84 vanagon had leaky heads when it got cold. Then one morning it had coolant in the cylinder and you don't want to hear that sound. It sat till summer and I pulled the heads and put on new gaskets and seals. Some heads pit and need replaced or repaired, mine were ok. It was a job as two sleeves were stuck in the heads and came off with the heads.

Jo Bo