There are so many ways you can go with the foundation of your house, just remember it is the foundation. The single piece that will hold everything together, the piece you never want to have an issue with. I chose a custom made trailer from TrailerMadeLLC located in Denver Colorado. The trailer was affordable and made for me. The studs are 24" on center and the flashing on the bottom of the trailer was included in the build. Three 6,000 pound axles make sure that the trailer can handle whatever you can throw at it.
For the insulation I went the spray foam route, it was easy to use and has an R value of 7 per inch. I was able to get an R value of 42 in the floor, I dig not wearing shoes in the winter ;)
Here comes the beauty of using a trailer that was built to suit a tiny house, no frame. The 3/4 ACX plywood bolted directly to the top of the trailer. Im into a little thing called over kill, all the pieces were glued down with liquid nails then screwed to the trailer with wood to metal screws. Incredibly solid and it doesn't feel like a trailer when your walking around in it.