Shaun,

2 large wheels up front and smaller in the rear on casters or vice versa would probably work just fine.

Hovv's plastic drum find is fantastic! The only downside I see to that is if you go to a rocky beach they may not roll well over the rocks, where a rubber tire would. It would be hard to get it over obstacles as well do to lack of traction unless perhaps you wrap them in a rubber sleeve? Like mouse pad material. or at least make a few wide strips around the circumference to add some traction.