If the house is not insulated well and drafty, I would worry about depending on warm air heating. Why not add radiant or BB to the new additon. Keep the duct work simple and designed for AC only. If you want install a HP to use in spring and fall, but go hydronic for the winter time. If your getting a seperate air handler for the newer part of the house and putting the hydro-air just there it might work. But tring to balance the whole house where 3/4 is still using BB and the rest needs hot air is going to be near impossible with a single air handler.