At that price you're not going to save much doing it yourself, plus fitting it in the van is difficult.
If you really want your RO in the van, this looks like an excellent deal.
Personally I prefer to have the RO in the shed and transfer the water to the van tank - less weight to carry around, less loss if you have your van stolen, trashed etc.
Purefreedom will supply either van mount or static, and if they're doing offers like this, I would go for it. I built my own system from scratch, but I still spent close on this figure.