Anything organic and the more manure you can get from stables etc., the
better. And well-rotted is best, yes. Whatever you plant give it plenty
of root room widthwise but don't plant it any deeper than it was in its pot.
You can judge by the mark on the stem left by the potting compost. Make
sure the roots are firmed in well when you backfill but be very wary of
stomping all over the newly infilled soil as some people are a bit heavy
footed! Firm but light is the way!
Maples like well-drained, moist but not wet, soil so you'll need to make
sure yours has a suitably prepared spot, out of the wind. Don't let it dry
out but OTOH, don't saturate it during winter especially, either. Most
Maples we see customers having trouble with are either in the ground and
desiccated by the wind or in a pot and both windblown AND drowning because
the anxious owner is worrying that it's too dry! But they're absolutely
beautiful things and you're obviously off to a roaring start in your new
obsession!

Great advise - thanks again !