colley, before you start can I recommend that you get a handheld TDS meter for less than £20 and use it to find out how high your TDS is?
Everyone seems to do as I did, start at the wrong end. Get the meter, slowly buy stuff and get your water down to zero. Then buy the back pack pole and a brush.
Honestly mate, a backpack is pointless before you have your water sorted, and the only real way to know how to get your water TDS sorted is to buy a TDS meter. (dont worry too much about water storage as I am sure you will figure something out)
I think almost all of us rush in to WFP kits as soon as we decide we want it, what happens is we always look back and think "I could have saved myself a few hundred quid if I had done this differently"
get the TDS meter. hunt around and see how cheap you can get one, its good practice.