I forgot what you said earlier, but if possible I would consider giving up on the portable building, pour a slab, and build a building from scratch. You won't be restricted by the access clearance and can make it wider. And building a shop is great fun! When I built my first building (20x20, for a shop) I bought some books to learn how.
Pouring a slab is not all that expensive if you do the forming and prep (rebar, etc.) yourself and pay someone to pour and finish. I've done several buildings here, the shop 24x62 and a recent one for a peacock house 12x24. It took me less than a day to prepare the peacock house slab for concrete.
If the site is not level, you can put in concrete piers and build your own wood-floored building, either on posts or as a portable building - exactly as the Amish building you've ordered. (Go to any place that sells these and look underneath and see how simple they are made.) BTW, if using piers to support a wooden floor building for heavy shop equipment I would put piers in the middle as well as around the perimeter to better take the weight - but a slab would be far better. If you know where the equipment will be placed you can even position piers appropriately at the expense of future flexibility.
Building a small structure on a slab is easy if you have mobility, can design and measure, and are handy with tools - and most people who play with wood can handle the tools. I've built a number of buildings, sheds, and decks here and at other places where we lived. There is also a tremendous satisfaction in building and working in a shop you built yourself! It is usually easy to find someone to help with things like trusses and setting large poles. I've developed ways to do most of it by myself using big C-clamps and braces, even when building with timbers so heavy I had to drag them to move them.
Alternatively, there are many people who will come in and put up a small building for you. If the site is prepped the cost may be less than you think and it can go up quickly (a day or two, depending on the design). Several times I've hired a builder friend from our church to build farm buildings and extensions or to help put up a roof - he brings a helper and they are very quick. He is willing to do small jobs like this by the hour instead of by the job which can save a lot of money since a contractor will often double or triple a job estimate to protect himself.
JKJ