I’m using a couple of shipping containers for storing work kit at the moment and through the winter they’ve been pretty dry – however I’ve also used ones that get pretty damp.
If you are looking at them theres a few things to consider
there are actual proper ISO shipping containers – ones that you could rock up with at a port and they’d accept it
and there are ‘Storage Containers’ – which are sort of shipping container like and are intended for used on building sites,
In the long run the ISO’s have a better life span, they’re made of Corten rather than mild steel so even when the paint gives up the steel rusts on the surface but doesn’t rot. Once a storage container starts to rot its difficult to keep on top of it – especially the structure underneath the floor
The former are usually better quality (at the time they are built) but some will have had a pretty hard life so condition is key. A ‘single trip’ container – one thats had very few trips and is in near new condition will probably set you back £1700 plus deliver for a 20 footer. Often 40 footers are only a couple of hundred quid more, but thats because the delivery and transport costs are much higher for them so they are less sought after.
Prices for storage containers (which a typically a bit less than ISO containers) tends to be quite flat, prices and availability for ISO containers fluctuates a lot and from time to time the stocks of them just vanish. When availability is good then less than perfect containers can be had for pretty much scrap value.
Some containers are sealed / air tight when you close the doors, the ones I have just now have small vents top and bottom so they breath.
Key to them being sound for storage of anything you don’t want to get damp is the floor being in good nick – leaks from above you can fix easily but damp coming up from underneath through a damaged floor turns the whole thing into a condensation machine. I’ve used one in the past that was non ISO and had a damaged plywood floor which I patched but the can was always damp inside. The two ISOs I used now have really good condition phenolic coated floors and so far they’ve been bone dry.