Built a "temporary" workbench of douglas fir about 20 years ago, I'm still using it. I made a quick vise of 3/4" pipe clamps, I still use it for clamping odd shaped pieces. Learned to use the jack plane, winding sticks, and smoothing plane flattening the top. If you want to make it mobile, I suggest casters with cam-type lifts so you can set it solidly on the floor for working. It's probably not possible to make a workbench too heavy. There's a lot to be learned by building your own bench, a very satisfying project.