Hi Andy,
purifing water is a straight forward process but you have to spend some cash. if you go for a di resin based system the inital expense is low, however, running costs will be alot higher. if you go down the R/O route your running costs will be less but the inital investment will be larger. the same applies with R/O systems. the more cash you spend, the better the quality of the kit and the longer it will last, further reducing running costs. how much work do you do?