Not exactly a conversion of a semi trailer, but we built a semi trailer about 4 years ago. We went 48' long (wish we would have gone 53') x 13'6". Front 12' is a lounge area with bunk beds and bathroom with shower and stool.
To try and answer a few of your questions:
2. Our interior and exterior walls are 2 x 2 square tubing. They were all welded
3. Insulation - we used rigid foam. If I had it to do again, I would have the trailer spray-foamed. I know it is more expensive, but believe it would be a better end result and tie everything together. We have .040 aluminum skin attached to the 2x2 tubing
4. Interior paneling - I tried to show some pictures. We have a variety of finishes. In some places there are cabinets, and the walls are unfinished behind there. Other areas we used the white aluminium and upstairs we used 1/4 luan plywood behind the indoor/outdoor carpet.
5. Ramp door - we constructed our own. There are 100 different ways to do the hinge and latches. We ordered some "airline" handles and latches from Featherlite and are pretty happy with those. The hardest part is getting the spring wound and set properly.
If you have plans drawn up, you are far ahead of where we were! We looked at a lot of different trailers at the races and took alot of pictures. It was a trial and error process, however.
As I'm sure someone else will advise you, there are tons of trailers for sale on racing junk, Hoseheads, etc and you can often find these at a steal of a price and put some work into them to update how you want. We had about 5 years of time (on and off around winters, racing seasons) to build ours. Always costs more and takes more time than originally planned.
Let me know if I can offer any other help - good luck!