How can I make tires look as though they are supporting a heavy load? How about a flat tire? Thanks in advance Alan

A lot depends on the type of tires you get in your kit.
If they are hollow rubber ones like in the Trumpeter LAV series you can put some putty at the bottom and before it dries press it on a flat surface.
If they solid ones try to heat them in hot water and use an iron to flatten the contact surface. This will be easier with resin tires than with plastic ones.

HTH
Olivier