Looking at some Elixir R brakes, are the calipers all the same, with a different mounting bracket to fit different size rotors? And if I order an IS mount front, can I get a bracket to fit this caliper to a post-mount fork?
(just doing some shopping now she's gone to bed)

Yes and yes

If you don't need the tool-free lever reach adjustment (who really does?) then the avid elixir 5 may well be a better bet (its probably slightly lighter having allen key lever reach adjustment)
Merlin are usually pretty good on brake prices, even if the rotor sizes available can be a bit limited

Cheers- looked like it should work, but couldn't see it on the Avid website.