You've got a couple of problems to deal with that will affect the wing loading.

Your size is going to be somewhat affected by the 40amp limitation on your motor. You power is going to be affected by the fact that you'll have to carry both a system battery and a power battery and you've got a maximum weight for the batteries. And you'll need to work out which kind of batteries, NiCd or NiMH to stuff in the plane.

THEN you need to figure out the steel bar load, and then how to lug that much mass in golf balls. It really looks like the steel bar mission is going to be what decides your cargo max load. You'll probably not be able to carry that mass in golf balls because they'll take such space that your cargo compartment winds up affecting the flight characteristics due to it's size. And everything has to fit in a suitcase.

So figure some weights and then think about wing loading.

A 40amp limitation on inrunners gives you some idea of the power of the motor(s) needed. Once you got your gross guesstimated, and an idea of your power, you can start fiddling the wing size.

What previous wing loadings worked in past contests? The most important one is going to come from the steel bar mission.