Some people on this side of the Pond have the same issue as yourself and what they do is store their tools in a Steel Shipping Container (not the standard side 20ft or 40ft length) but the smaller 10ft unit though the 20ft unit is better if timber or scaffold poles are stored and some are fitted out as offices too or even customised with racking.
There are Aluminium bodied containers But I won't recommend them because they can be entered
by hacking through the panels and B) they are not suitable for flammable items
They can be rented out on contract. You go to the yard and have a look at the choice and Most rental companies deliver them direct to the site and offload it and collect it (all in the price) Query the purchase price too I think its about the same as a cheap car (New? about the same as a car)
And they'll drop it off as part of the price

This link is for a UK company, and it gives a rough Idea of what You get for your money (have to convert to dollars by multiplying by 1.8)
www.sjonescontainers.co.uk/saleshire.htm I expect they'll be listed under Shipping services or Shipping Containers in your Yellow Pages And they'll situated on the Docks or a freight yard
PS Some I have seen have motion alarms fitted