I did the shipping container after looking at a lot of various options. I couldn't build as large a structure for less $. And have it water tight, moderately secure, and mouse proof at least. My machine is smaller and fits just fine with ROPS up. But I will add this:
Cost for a 40' vs a 20' is just a small % more. Unless space is an issue, may as well get a 40'
It takes very little for ground prep to put one in place. Somewhat level is about it. Best option for me on fairly level ground was 4x4x8 concrete cap blocks. Just one under each corner point is all that's needed. I also used the smaller solid concrete pavers to level. Need solid blocks - cinder blocks are likely to crack.
For entry, I ran a 2x6x8 on top of the front corner blocks, flush with the corner pieces. This leaves a little overhang. I screwed 3' pieces of 2x10 PT onto that to make a ramp. That fits perfectly under the closing latches yet isn't a high enough ledgde that a tractor or ATV can't easily climb it. A basic riding mower can't & needs help in.
Leveling the container on blocks (or beams if you prefer) is important. You want left-right level and a very slight slope back to front. The left-right level is important so the doors open and latch closed easily. Unlevel and the doors can bind and be real hard to move. The slight slope toward the front is in case a heavy rain shower catches you - you wouldn't want the rain blowing in and running to the rear or otherwise not draining from the front.
There are various grades of container from "one-trip" (basically new) to pretty much unserviceable. There are grades in between. Most of the cheap sellers on CL are yahoos with an F250 and rollback trailer. They don't have the container. You order, they go to port and buy something that isn't suitable for another ocean trip & bring it to you a week or two later. No idea what you're going to get. I ended up getting mine from a local leasing company. It was still sight unseen, but was a container on their yard and something they would otherwise keep in service. Guaranteed to be free from holes, have a good floor, and be water tight. Not nearly new condition, but not scrap metal either. It was about $100 more and well worth it to know if I wasn't satisfied I could turn the driver away and they'd bring a replacement for free the next day.
As long as there is plenty of maneuvering room delivery is surprisingly simple. Mine came on a big truck with a rollback trailer. It was unloaded, leveled on blocks (that I had to provide), and within 1" of where I told them to drop it in all of 15 min or less. Once in place - or if you just want it dropped and worry about leveling later - it's simple to jack up one corner at a time and level the thing. They're heavy, but only lifting a corner at a time doesn't take much of a jack.
You do want it off the ground a little. I found a lot of the clist sellers saying it didn't matter - just drop it on the dirt. But after talking with people who make their living with these things it was clear that a little room for air circulation underneath will avoid lots of interior condensation problems and extend floor life. Mine is 8" off the ground in the back and 5.5" in the front. That is plenty.
For tools, tractor, air compressor, generator, 4 wheeler, etc it is very nice. For implements however it isn't a good solution. Whatever is on the 3-pt sure. Otherwise the width is an issue even with my 5' attachments. Since you only have 8' width, items can't "pass" each other inside. Unless you want to cut an additional side door or get the kind that opens from both ends, you could find yourself needing to unload the whole thing just to get the implement in the back.
