This great, get it done, don't let it become the mother of all sagas like the LHR third rwy fiasco (yawn).
Will it be a dedicated shuttle LTN-Luton Parkway, or will trains continue to other destinations e.g. London, Brighton, etc.?
Perhaps the arrival point could be easyJet HQ. Hasn't there been all of them potentially relocating in Luton outside the airport? How much do they use the hangar for actual plane maintenance?
Don't really know the airport, but the arrival/departure point needs to be as near to the terminal as poss. If spending the money, make it as convenient as possible for pax (especially those with luggage).