This is typical "Pay me high now, or pay me more later" situation.
Unless you protect the black tank from UV, you can expect about 3 years before it will deteriorate. Ask manufacturer about warranty.
I would not count on spring water. Not likely you will be able to storage it as it will grow algae, when the creek likely is protected.
If barrels don't work for you for fresh water, farming industry offers lot of different water tanks. They might not be drinking water rated, but I would not drink water who sit for weeks in tank anyway.
You put the tank on a hill, or elevated by construction - you'll have gravity feed, what will save you battery power for pumping.
Good luck on the project.
