Delivery

We have a single constraint on the mass we can deliver to the surface intact, and that is Mercury's escape velocity. Even if we can get stationary above the planet we have to fall and stop. Falling from stationary hits the ground at 4.24 km/s and all of that energy needs killing by engine since there is no atmosphere to brake with. We can perhaps get to stationary without any thrust beyond earth escape velocity, but getting down intact has this minimum unavoidable energy requirement.

Initial limits:

maximum acceleration: 43m/s^2, braking to zero in 100 seconds.

create a graph to a soft landing. Killing all the speed as late as possible takes least fuel , for a given engine. The highest thrust per gram minimizes the mass of propellant.