I think a lot of people are overestimating the material needs. 2 x 2 x 1/8 wall will be fine. Gussets on the mounts but not necessary on the corners unless you're a crappy welder. Good casters for sure but pneumatic would be much better to dampen the vibes. Unless you're anticipating running an extremely unbalanced engine or have a good slope on your floor or driveway there should be no need for "feet".

But its only an opinion - not the start of an arguement.