Water quality and PPM

[Longroots]

I didn't plan to grow my collection into more sensative orchids but I have and I've been gathering a small amount of rainwater from a place it gathers in my garden and filtering it through coffee paper to remove debris.

I have a water butt but I so rarely use it I worry that the water in there now is not good and there will be a lot of dissolved salts and other things from the rod of my house. The roof has a fair amount of moss on it so water has travelled past and through parts I'm worried will increase the PPM of the water I gather. I cannot afford the space or the cost of a RO system, so please do not suggest this.

Primarily I want to know if simply emptying the water butt and letting it refill will be fine for the plants I have or will have (Masdevallia Measuresiana and Dracula Lotax). And the suggested PPM to have for them when using rainmix fertalizer. I'd also like some recommendations for decently priced TDS measurers for my water in the UK.