All good thoughts.If you set it up properly, glue your PVC well, use good thread sealant on your threaded fittings and make sure your copper tubes are well connected, then you shouldn't ever have any trouble.
You mentioned that you didn't want to have all the tanks up and running at once so you could run all of the PVC (since it's cheap) and use PVC plugs in the spots you don't currently want to set up. Then, as you go, you can buy the fittings for the new tanks and spread the purchases out over time.
If I want to keep it cheap how do you feel about the 3 stage filter, cheap irrigation 90s then use irrigation valves like these? Other than the filter I could do the whole thing for probably $20.
https://www.amazon.com/Orbit-67404-...953&sr=1-2&keywords=1/4+inch+valve+irrigation