People have built shops out of sea cans (shipping containers, storage pods, etc.) already and they turned out quite well. I remember seeing photos somewhere from a fellow in Australia who did exactly that. It was a bit crowded and tended to get warm inside but it was dry and secure!
Myself, I would save the sea can for storage, as you will have plenty of need for that too. Let's face it, you're going to need a small tractor of some sort to mow your acreage as well as some other garden type tools, and an 8 x 20 would be perfect as a large 'garden shed'. I would build the shop on a nice heavy concrete pad, just make sure you build it bigger than you think you will need. Every time I talk to someone that has built a new shed they comment that they should have built it twice as big as they first thought, or that they should have gone with the 14' walls instead of 12', etc. Plan carefully and think big, you will not regret it.