I should have included toilet chemical as well,
I would suggest that you just get the Blue cassette waste tank chemical, forget Thetford Pink for the flush tank, it causes more problems than it solves.
I would also suggest that given the option, go for Propane as it will work in all temperatures unlike Butane which stops supplying gas at around 3 or 4 degrees C.
Depending on the age of the van you are getting , if it has a bulkhead fixed regulator you will just need the correct "pigtail" (gas connection hose) for the type of gas you have.
If you have the older van with cylinder mounted regulator, you will need the correct regulator for the gas being used,.