TomW,
This is where I come to learn, and help when I can.
Why the gravity feed?
How about two tanks, upper is small for daily flow.
Lower for the mas storage. Pump to upper tank when you have the power.
Ebay plastic potable water tank 2500 gal for $1000 w/ship.
I will use one or two for potable water. Rain water.
EPDM Liner for the heating tanks. What did you want to pay?
In ground was cheaper, to make an above ground tank, surely it will cost more than the pump?
Consider an above ground pool as a storage tank, used pools are cheap.
Liners are not too costly or use EPDM liner for heating.
Pump, I am sure you have looked at RV pumps?
Ebay: 4.7gpm RV pump for $50 w/ship. Works great on my well.
Not sure how long it will last, couple friends have been using them all summer long for years, still working.
Have fun,
Scott.