1.5 ton should be enough to lift a whole disco axle clear of the ground, or one by one sill to get both tyres on that side clear. The jacks rating should already include a safety factor. 2 to 3 tons should be more than enough, any more and the jack will just be too bulky/heavy for what you want.
Not such a problem with smaller torlley jacks, but most new trolly jacks have no way of locking the handle, which makes manovering them a swine. Likewise I don't like the trolley jacks which you have to twist the handle to work the loering valve. Neither of these problems will prevent a jack from working, but they do make the jack less user friendly.
