I have always felt that the best demo match was Seamus vs Justice. You get strong melee, strong shooting, plenty of casts, an intro to summoning, board control, terrifying, bury, and + and - twists. All with out upgrades; plus a few other things (Finish the Job, for instance).
Personally, I would say to keep the set simple and the cost down. Models, rules manual, and 2 decks. Maybe include a sheet with tokens and blasts to cut out. Getting the set out at or under $100 seems important to me.
Easiest way to get current players to buy it is alt sculpts of anything, hands down.
Alternatively, making a box containing just extras (tokens, cards, card terrain, etc) but no models could be viable, BUT defeats the purpose of a two player starter (one box to buy to start the game). It would still simplify things, though.