I have been kicking around the idea of a toyhauler for too long? My idea was to buy an 8x20 rear ramp door trailer and fit the interior out to my design. That way you are starting out with a stronger frame design, 3/4" flooring, the built in ramp door and lower load angle. After looking at the pros and cons I decided to look for a good deal on a used toyhauler? and I am still looking. If you wanted to build one to suit you, I think I would start with a rear ramp car hauler. Just my 2cents.
Good Luck
Craig