I vote for: you're older. Hire some high school students to finish at
least the digging for you - be done in a couple days and they work
cheap. I just built my GH last summer and it almost killed me. But you
have the right idea about the insulation. I put the insulation panels
all the way down to the frostline, 32", when I built mine. They come
even with the top edge of the block foundation, 8" above grade, and are
capped by the oversize sillplate which covers them and the block. This
entire mess is then faced with a panel of vinyl siding capped with
flashing to give a nice finished look, protect the insulation from
damage, and to allow the rain to run down over and outside the whole
assembly. This keeps the foundation from sucking out all the heat from
the GH. I did not insulate beneath the gravel floor as I only heat to 55
degrees in the winter. Thats's all they get. Usually our winters are
clear enough that most days it gets into the upper 60's in the GH. I
turn the thermostat up to 70 on watering day and the day after. With the
triplewall glazing, the foundation insulation, and being attached, it's
much more affordable to heat, which was indeed the plan.