Moose, add a little extra length, or width, to house your implements. I presume your gardening persuits would require the addition of a tiller, and that mile long road you are dumping gravel on would be a lot smoother with a blade or two.
As for the metal carport things, I looked at some of them and found that you get what you pay for. Some cost a lot more than others. I even found one that looked like a barn, came in colors and had full sides and roll up doors just like a stick built barn but could be erected in a day. I think you need to figure out how much you want to spend on the shed and then go look at several dealers of the prefab steel buildings. Look at how they are fastened together, the weight of the steel, the options they have (some can be purchased as a carport and later EASILY enclosed, some are designed to remain an open carport and can NOT be easily enclosed).