Hi
I don't know that much about these things as I'm also new to wfp but CaCO3 is undisolved calcium which usually, I think, means chalk and the Ca bit is the dissolved calcium. Basically it is how hard/soft the water is.
7 degrees Clarke is 100 part per million impurities (ppm) and that is the important bit.
Your water is roughly zero to 400 on a tds meter and more likely to be always nearer to 300 which is about the same as where I live.
I looked into the Ionics resindentual kit and decided to wait untill we could get an RO unit purely for the same cost reasons you wrote.
Also, our customers were by and large happy with us just using tap water anyway and I'd no way of knowing how much difference the resindentual kit would make apart from one thing - it would cost a lot to run.
My advice would be to wait untill you can get an RO unit although I may be completely wrong about the resindentual kit in hard water areas.
How we cleaned conservatory roofs was with a 2 gallon pressure bottle (the one with the pump handle) first with an 18ft wfp brush and good old fairy and then rinsed with tap water. They didn't come up perfect but they came up ok.
As for how long the resin lasts, only a tds meter can will tell you that after you've spent the money on resindentual kit and used it.
As for quoting, what I'd do is go a bit lower this time because you do not have a wfp system yet and explain that and also explain that when you get your wfp system, the price will go up a bit. Once someone has seen a conservatory roof done with wfp, believe me they will pay that bit more.
As a rule, when we quote for consevatory roofs it is always a bit less than for doing the house ... within reason. So a £20 house would be £18 for the conservatory roof.
This is not about w/c'ing but is worth a read anyway -
http://www.modeleng.org/articles/hwater01.pdf#search='degrees%20clarke'
I hope that hepls a bit.
Cheers