From a different perspective, have you tried sounding out current warehouse owners near you? Someone may have spare space or a mazzanine available. There could be a good working relationship for both parties. I wish I could sublet some of my warehouse to someone like a book seller but the lease prohibits it so you're probably looking for outright owners. The idea of letting out one wall 20M long, 3M high and 30cm deep that I wouldn't miss in exchange for rent that would buy a weeks holiday a year sounds appealing to me and could be a lot less hassle for you, I'd even throw in free use of my stepladder.

Whatever you do look for a modern unit/storage which will be insulated therefore slowing down the day/night temperature swings as well as through weather patterns and changing seasons. A well insulated unit/storage facility (as well as using wooden rather than metal shelving) will greatly reduce the risk of condensation ruining your stock. Trying to heat/cool or even frostproof any storage facility could cost more than the rent. Don't forget about other bills such as power/water if connected.

Check any lease carefully no matter what the size of the storage as commercial leases are a world apart from domestic property rentals, service charges and even drainage charges for the rain that lands on the roof are common even if you don't have mains water connected, and that's long before you get to delaps.