Hi Mike,
Ignore what that person has told you, by the sounds of it, they are just looking to exploit money from you.
If you are in a hard water area, you need an RO unit. The running costs are very low, possibly $60 Dollars every six months. In addition, the RO unit will remove up to 99% of impurities. I would still recommend investing in your own DI bottle, you should be able to get one plus 25lt's of resin for about £100 dollars in the states.
BY using the RO unit, then using your DI bottle as a final polisher, your water will be 100% pure and your resin will last months.
Hope this helps
Andrew