Hi.
If your water is soft then a DI only system is your best bet. Its simple and I am envious
My advice would be to go for the biggest tank you can fit in your vehicle. I know you say one using wfp ustairs and one trad down but I bet you it wont be long before you will want to do the lot WFP.
Start with a tank, pump, flow controller, DI vessel, good quality reel and 8mm hose, and suitable poles. DIY will be very easy to do for that set up.Ebay is just like any other market place. There are good and bad there. Avoid using an unbaffled IBC tank in your vehicle like the plague. You see these pop up on Ebay masquerading as systems when they are more like death traps and are very highly overpriced for what they are.