I was advised to not go as heavy as I planned with 4x4s so I have been planing around with a few designs and I was wondering how robust my table should actually be?
Is 2x2 mild steel enough to hold everything?
I was thinking a 5x10 table with 3 sets of legs
How thick should the stock be?
Is 065 to light?
For a large table like that, don't skimp by going with skinny 2x2 tubing for the table frame.
The answer to your question would be go as robust as your budget and fabricating skills will allow. Many would also recommend doing FEA analysis of the design, stress relieving the steel after welding etc. Look at other similar builds here on the zone and then follow their examples. There are many successful builds here.
How much time do you have and what facilities do you have for correcting any distortion?The project may not be easy,but it isn't necessarily impossible.For inspiration you can do a lot worse than studying the spaceframes that support some older aircraft engines.They require good fabricating skills and rugged jigs for assembly.Alternatively you could get hold of some hefty top rails and use the smaller stuff to hold them at a convenient working height.Sharp acceleration and deceleration would probably cause the table to vibrate a bit unless attention is paid to damping and triangulation.As has been mentioned,there are FEA programs to help you and some of them are even available at no cost.