Thinking about building a cabin. Advice?

Yes, it's certainly an odd ball trailer, only one I've ever seen with that axle configuration. We bought it 15+ years ago from the RM, they had used it to move their dozer around, and that was the main reason we bought it. We eventually made a water tender out of it, but it was a nightmare keeping tires on it, it would just eat them up. We no longer use the dozer as it's tracks are shot and bought a better water trailer. For the last little bit, instead of fixing the tires in the center I just chained it up to the frame and seemed to go through less tires that way so figure the center tires were causing an issue somehow and causing the flats, maybe because of the crown of the gravel roads?? Seeing as it won't be moving much and only behind a tractor, I might just chain up all the centers, not like there will be a pile of weight on it. Will reassess that after it's done.

I don't know what kicks around on the cheap used market for windows, were you looking for the older style double sliding window with a screen on the outside of one half of the window opening, or a more modern double pane window with a full window area screen on the inside and a crank system to open the window. I would imagine if you can find whatever might be a common or "standard" window size would be ideal so that in the event of a breakage another unit could be sourced rather then sticking in some one off window size that would not be easy to find on the used market in the future. I'd assume you would want screens in case you use the building during the summer.

You should build two while you are at it, a couple at RB in Edmonton. Sitting at 19 and 21k at the moment. Make a few bucks on the second one to pay for the first.
www.rbauction.com/CUSTOMBUILT-16-Ft-x-12-Ft-Cabin?invId=12964590&id=ci&auction=EDMONTON-AB-2021176

It would be interesting to know what the cost is of the lumber and a fair amount of that appears to be 6x6 that goes into one of those builds. It sure doesn't look like a very roomy space though from the photos and that ladder takes up a lot of open space. As to insulated value, I would think a well built insulated wall is going to do better then solid lumber by a fair margin. The downside of insulation is any rodents etc that could get into the walls. Were you going to do any wiring in case you wanted to run a generator sometimes and have lighting and outlets for power.

I would get some batteries and solar panels, suitable for whatever your needs are, and an inverter (12VDC - 120VAC) on sale they can be only a couple hundred $$.
Could do a combination of 12VDC and 120VAC lighting and appliances depending on what you want to accomplish. The inverters are nice but they do waste some power.

I've heard that comment before about lofts being hot, very hot depending on the heat being produced. I remember years back I stayed at a friends place in BC in an actual log house which was on a basement as well, anyway I stayed in a bed in the loft area and oh was it warm and yet the rest of the building was a normal temp. In any event good you are planning out some electrical and plan ahead for solar as well and a proposed spot to mount the electrical boxes as one idea I was thinking of was to not take up any room on an interior wall with a breaker panel and inverter etc etc but build a steel box to mount on the back side of the cabin, a water tight box ( like some grain dryers have for the main control box ) . I like Optimal's idea of running a separate 12 volt circuitry so if you did decide to set up solar you could run efficient 12 volt LED right from the battery like RV's do. Wire in for a ceiling fan and in fact depending on the fan perhaps a 12 volt fan to again emulate the RV situation if it moves enough air to get the heat out of the upper spaces. Use older tired batteries from farm equipment that can still do the job but are not the best and the solar would keep them charged as new deep cycle batteries can get very pricey for what they are vs a row of used group 31 batteries for example.  

I thought about doing a 12vdc/120vac combo too. Not worth the effort and cost. 2 deep cycle batteries and 2 100 watt panels are all we need in summer. That setup will power a box fan sitting in the window for poor man AC and plenty of juice left to make toast in the morning. Generator is pretty much for the toaster oven if we feel like making a pizza or something similar. I will have to get some pictures or video of what we have going. I do need to build my solar tracker too one of these days.  I should note, don't piss around with old tractor batteries, save them for grain augers lol.