The absolute minimum power to run out would be 20 amps 220v 4 wire. This would allow 2 110v circuits and the ability to run a 220v circuit if necessary.
I personally would run 60 amps 220v. You never know what you'll have out there in a few years. The only additional cost is the difference between10 gauge and 6 gauge wire and the larger breaker panel.
You're going to have a great storage building shortly.