Hi Tom
I would get a hand held TDS meter and check the hardness of the water in your area. Depending on the hardness might determine which route you go down for WFP. After you have done this, check out ebay and this site for second hand kit. For £3.5k you will get a pretty good kit to set you on the way.
Size wise, if working on your own a 350 litre tank is ample, I would not look at anything smaller.
Cheers and good luck.