Tables: you must have plenty of space!
Realistically, if you're going to do that, then the heavy duty welding table will do all you need. Use 50x50x3mm box section for the legs. Make the top frame from 25mm box, with angle iron diagonals. Weld the legs to the frame ensuring that everything is square when finished(which is the hard part). Add a similar frame about 50mm up from the bottom, and panel it with wood as a storage space. Have a piece of 6mm steel cut for the top, and select as flat a piece as possible. Bolt it to the top frame along all four sides and the diagonals with countersunk bolts every 50mm. That shouldn't tax your welder. It will .cost some serious cash, which is why fabrication shops save them for the precision jobs. I use an offcut of 10mm MDF on top of the chest freezer, or clamped to a £20 folding workbench.
Vices: a fabricated vice is possible but will require machining to be any good, and will still be light duty. £100 at Machine Mart will buy a useable, large engineers vice. I would look for a secondhand 6" or bigger Record, which are out there and a much better deal.
A trailer doesn't need to be that hefty either, take a look at one of the commercially available ones.