I have been considering a similar idea, putting a garden far from house and source of water. My thought was to build a small outbuilding near it. For two reasons, the first being to have a roof to collect water off of and the second being to store tools in. With the garden being far from the house water is not the only thing I would have to lug to it, so why not kill two birds with one stone and have a source of water and a storage area?
Builditsolar dot com has plans for making water storage tanks out of plywood and pond liner. As long as your pond liner is not pvc you could use it in side the plywood box. One thought is you could build the tank right into the wall....get it off the ground for ease of drainage... I had thought about getting a (used?) heating oil tank for storing water. Math said approximately 1/2 gallon per square foot of roof per inch of rain. A 12 by 10 building could provide 60 gallons per inch of rain.
OHHH and look into the hugelkultur threads, water storage at the roots of the plants via buried logs sounds ideal for your use (and my potential use too
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