Kind of late to the party, but here's my 2 cents worth. I have a 36'' door - I'd go with the 32'' if ordering. The 36'' is hard on the hinges etc... I'me 6' tall and 225 lbs and I actually don't have an issue with a 24'', except when moving things in and out. An RV door is nice in that you can have a standard screen door, but I just added an RV latch to my standard door. I also added an extra latch for the standard lock bar to the outside so I can lock it open and not get locked inside. The reason 36'' doors are required for entry in houses is because of wheel chairs. You're not likely to be taking a wheelchair through the side door. If it came to that I'd order the rear door as a ramp and bring the wheel chair in that way. Speaking of the rear doors, if you don't need a ramp order the barn door. That way you can open them part way for a breeze, or only one, but you don't have to have the back wide open to enter that way.
Axle - if you're spending almost all of your time on paved roads the torsion axle does ride better. They can be ordered with a higher start angle if you want the trailer higher, but that has to be ordered from the get go - can't be changed later without changing the axle.
Height, my trailer is a little odd, as the roof is rounded. The walls are 6' tall but at the center of the finished ceiling it's 6' 4''. I find that plenty, but if you're prone to claustrophobia you may want the extra 6''. My door is about 5' 10'' but I never hit my head as I'm always stepping down out of the trailer.
Windows, I have 4 small ones including the one I put in the door. 3 open for ventilation and I have a fantastic fan in the roof. I like the small ones for security and lack of heat loss area, but when confined inside due to inclimate weather it would be nice to have larger windows to sit in front of.
Insulation - most factory insulation is nothing more than Reflectex over the ribs and under the plywood. It probably works fairly well at keeping it cooler in hot weather, but is only a little better than nothing in cold weather. My walls and ceiling are filled with 1'' styrofoam and it seems to work well, with the exception of if it gets really hot inside, the walls take a long time to cool down at night. This is really only a problem in temperatures over 100 degrees.
Electrical - most trailers won't come with a battery, they get their 12 volt from the vehicle plug. This is not a big deal to add to the system. If your needs are minimal then just have the manufacture put a light on each side at the rear, and one centered in the ceiling straight in from the side door. The fixtures will be junk, but those are fairly easy to change out. It's the wiring that usually gives people fits. If they can add a couple 12 volt plugs in the front that would be nice also. Definitely get them to put a powered fan in the roof - it makes a huge difference in how fast you can cool the air inside. Also add a second vent if that is an option. Having an intake and an exhaust at opposite ends of the trailer is a big help.
Make sure you get a spare tire and wheel. The tires will be junk as others have said, but you can change them later.
The rest of the interior can be fitted out with a folding bed, or just a mattress on a simple platform. That and a few plastic drawer units, and hanging units screwed to the wall like shoe holders for a closet door, and you'll be set.
Now that I typed all that, I'd encourage you to look for a used trailer that has most of your priority needs already. At least around here they are about 1/2 the price of new for a nice trailer.
