I have a sporterized Lee Enfield #4 carbine that is .303 british caliber. It is a solid shooter and is a decent caliberfor hunting deer and similiar size game. Ammo does not tend to be pletiful, but is available at places like Gander Mountain, Sportmans Warehouse and Cabelas. Depending what model andcondition of the 303 (I assume it is a Lee Enfield)you refer to, you should be able to get it fairly cheap, almost certainly under 2 bills.
Comparing performance of the .303 to the .30-06, the .30-06 isbetter.