I agree that a 1/4" steel top sheet is the way to go, size it as you think you need for your size of projects.
I recommend using a hand grinding wheel on the whole top surface of the table for two purposes- first to get the scale off for better conductivity, but just as important, this operation puts some texture on the table, which makes parts tend to stay where you put them rather than sliding around when you're trying to weld them.
I welded up a subframe of 1-1/2" angle iron to support the 1/4" top with 1" square legs to the floor and 1" square tying the legs together about 8" from the floor to stabilize it, except in the front where my feet go. If its not strong enough or sturdy enough after your first shot, just weld in some more cross pieces until you're happy with it.
On mine its easy to remove the top- this allows you to put large irregular pieces (auto rear ends for example) on just the subframe for welding.
You'll find that you'll customize your table as you go, adding holders, etc.
Making the whole thing more portable- other than making the top easy to remove, I guess you could make parts of the frame bolt together, or have the legs bolt on to the upper subframe but if you really want it to be sturdy I'd avoid that if possible.