if it is a great table in good condition, it would not have a 1/2" sag in it,......sometimes "free" isn't necessarily free. my thought is that if the table has sagged, adding the weight of leveling compound is probably going to make it sag more. if you have to reinforce the table top to add the weight of leveling compound,..... you might as well consider building a new table. solid core door blanks with small defects can be had fairly cheap from building supply brokers or big box stores ,( look for one with a chip in the corner or a miss-bored lock hole) any small defect and they make excellent building tables. build a 2x6 frame around the door about 1-1/2 inches in from the edge of the door with a couple of intermediate cross ties, legs of your design or choice and you have an excellent sturdy new table for very little money. legs can be as simple as two 28 inch long 2x6's, screwed together in an "L" and bolted onto the inside of the surrounding frame. carriage bolts work good here because they can be easily removed if you need to move the table at a later date.
I have one such table and it has served me well, for many years.
if you have a dedicated room as your shop, a couple small corner brackets will secure the table to the wall and make it rock solid,....nothing better than a table that will not move or wiggle when used ! all your wall will suffer is a couple small drywall screw holes that can be easily spackled if you move the table. make sure you hit wall studs with those two screws.