Since it's a pistol, the biggest caliber you could legally use would be the biggest caliber you could find. Also since it's a pistol, I'm not sure you could use a 12ga because that would(probably) be an AOW, which might not be hunting legal.
There is no limit to pistol calibers when shot from a pistol. A .243 sounds like a fine choice. I don't know enough specifics to say how far out it's effective on deer, but i guess it depends some on how accurate you are with you handgun.
I just looked up the law to make sure. The law says the round must be .243 or larger, not muzzle-loading(different rules), atleast 4" barrell, case over 1.16" long unloaded, and no full-metal-jacket bullets.
http://www.in.gov/dnr/fishwild/files/fw-Deer_Hunting_Equipment.pdf
you can get a 30-30, 30-06, 44 mag. for the contender pistol. the 44 mag is fun to shoot.
I shoot a 45-70 super 16 contender and its legal in hand gun form.
45/70 is not a legal round to shoot out of a rifle. It is legal to shoot out of a handgun.