In terms of creating a mesh from spot heights, i don't think there's an easier (within archicad at least) way than adding the points to a mesh and manually adjusting the level height to suit. i would think with enough spot heights, you should get a reasonable site mesh model.
in terms of creating accurate contour lines for the mesh, you could try the technique outlined here:
http://archicad-talk.graphisoft.com/viewtopic.php?t=27936&start=0&postdays=0&postorder=asc&highlight=&sid=b3dee3aaf8bd3b2a5affc14ea905f33f