Nice job ID, I have plans for a similar structure project of my own. I don't have the kind of logs available to me as you do, I have found a good alternative to use for making the walls out of.
These are a concrete block that a local company builds, we see a variety of these blocks used in our area primarily in gravel yards. They use them to separate different sizes of gravel, rock, you name it. The one I am using here in this picture have a decorative texture on the outside surface.
The blocks are 2 feet wide, 2 feet tall and 4 feet long. They have a V shaped ridge on the top and one end and a corresponding V shaped groove in the bottom and the other end. This makes the blocks lock together. They weigh 2500 pounds each so it takes some fairly large equipment to handle them.
The beauty of these is there is no need for a poured footing. We use a laser level to build a flat grade to set them on. We compact the soil by driving the tractor over the footing area several times, water it down to settle the soil and compact again. you can use a thin layer of sand to create the final grade. Once you get the first course of blocks lined up, the rest stack on top very easily.
For water proofing, we use a heavy roofing tar to smear between the layers. We trowel it on in a row about 3 inches wide and an inch tall, once the next blocks is set tar pressed down and created a nice moisture barrier. On the outside surface we use a sandwich layer of 30# roofing tar paper and 10mil plastic sheeting and more tar paper.
The wall shown here is 20' long and 8' high, you could stack then 5 or 6 high without worry. There are two ways we go about the roof on the structure. We have used bar joists to span the blocks. We use concrete lags to secure the joists to the blocks and then lay 2 layers of 3/4" plywood over the joists and screw them down with self drilling machine screws. You could also use 2x6 or 2x8 or 2x10 boards on edge depending on the span and soil load you plan to put on top.
My plan for the equipment shed will be these blocks on three sides like you have done with the logs. They will be stacked 5 high creating a 10' tall wall. I am collecting used telephone poles to span across the blocks and then use Sepp's method of covering the roof logs for waterproofing. I plan to use one log on each side wall laying lengthwise to the block. It will rest on the block at one end and be held up 6" at the entrance end of the wall. This will create a slope for the cross logs to lay on and give us better drainage off the roof.
I am using these blocks for a variety of projects. I am building a greenhouse this winter where I use a row of these blocks down each side and across one end. I have fabricated steel support brackets to bolt the greenhouse pipes to the block. I went this way for 2 reasons. The first reason is it saved me having to drill and pour cement in 18 holes to anchor the posts. The second reason is we are going to use the blocks as a means of passive solar heat storage. The outside and top of the block will be covered with 1" board insulation and 10mil water proofing along with several inches of soil. The inside wall, the textured side faces into the greenhouse. The idea is the block will heat up during the day and then give back that heat over the night. We can lay a sheet of insulation against the inside wall in the summer months to shade the blocks and not overheat the structure.
The blocks cost me $36 each, I pick them up. So the wall shown has $650 worth of block used. This would not work for everyone but for my needs they work great, relatively cheap, easy to stack and incredibly strong.
One more comment to make on lighting the tractor shed, I have sourced an excellent 12V LED light that we use in other areas of our farm. They are a 720 lumen lamp, they draw very little power, I have a row of 10 of them mounted in the ceiling of a storage container and the total current draw of all 10 lamps is less than 2 amps. You could use something like this screwed to the ceiling of your shed, mount a small 20-30 amp battery and a 40-50 watt solar panel to charge the battery. You could run 10 of those lights on a battery that side for 10 hours straight with no problem. This method would let be independent of your normal power system and would work great for off grid or BO locations.