Hi Geo, obviously we can assist you with affordable (and appropriate-sized) panniers, just swing by the Store - we are open - and we will show you 4 or 5 options...
Regarding the tank, my suggestions as follows:
1. Will not myself recommend storing water in a tank previously used for fuel...
2. Customer on here poss. Looking for a RADE underseat tank...
3. You could sell it and fit the cubby hole in its place - retaining the RADE air box, etc, and then simply lie a water bladder/ camel back inside the cubby, then simply take the bladder to the tap or water source, etc.
Husky shown here, but we stock for KTM, price shown is WITH air filter, obviously you need JUST the ‘cubby’, will forward price tomorrow.
https://flyingbrick.co.za/product/rade-garage-under-seat-storage-kit/I DO think tricky getting water TO the fuel tank, in remote places, the bladder is a much better option, and of course would not put water ina previously-fuel-containing tank.
Cheers
Chris