34,000 to 40,000 is way too much to pay for a used truck. I used to build on and own FL trucks and I had the occassion to own a Volvo VN64. I'm completely sold. Quieter, more comfortable, better aerodynamics and it just feels like a beter truck. You can get good used units all over the US for under 25K. The common combo is Detroit 60 series with Eaton or Rockwell trans. Both adequate in my humble opinion.
Depending on where you live and whether you drive in winter, I would recommend either aluminum or steet framing with minumum or .040 aluminum siding.
As for interior finshes we use a hardwood plywood exclusively. The 'printed" luan is impossible to repair if it gets scratched and the plywood we use paints like sheetrock and it therefore easy to repair.
There are a few ideas. The people on here are a wealth of information. You will get shat you need on here I'm sure. If you have any more questions for me, feel free to contact me.