Good questions......

I regret buying used...... Not that it was a bad sell but I had to replace filters and membrane not too long after and by then I could of had a new system..

Also get a inline tds meter or a couple... I have a dual, it reads my incoming and outgoing. I then have a single reading after my membrane to tell me when it's time to change the membrane, but mostly to monitor the tds going into the di resin canister...

I honestly just wished my filter would last longer.... But I like to change them right when it hits 1. I had a cyano bloom when I let it slip passed and will never do that again..

mine was a 100gpd or higher but changed it to a 75 I believe....