I use 2" brush for corners and mini roller for the rest . Be carefull how you buy your chosen system as it can be better to buy liquid and tape separately as the old wp1 kit had liquid, tape and sheet , I found it better to buy separately .
Yes the wp1 system your right about how you buy it but the tank it, I think you need a complete system. I’ll scout about online see what deals there are. Not sure if the lovely forum sponsors stock either