Hi Hazcop,
I found these but I am unsure if they will help.
1)
http://www.aquariumadvice.com/forums...on-100293.html
2)
http://rockymountaindiscus.com/rofaqs.htm
3)
http://www.peteducation.com/article.cfm?articleid=2904
From what I understand from reading the above, you can use bucket so it doesn't attach permanently to yr sink. Use a super long tube.
Instead of a reservoir, you can use a bucket. It depends on how much RO water is produced.
Rain water is soft. Do u have a rainwater tank?
I really don't know much abt RO. DOn;t just take my word for it.
Its best u make time to read the articles and comprehend how it works.
All the Best.