Any 1,000,000candle power or higher spotlight with a red filter is fine for 300yrds. Typically between $30-50 at any hardware store, or even wally world. Rechargeable is best, and you really do get what you pay for. A $80 light tends to last a lot longer than a $30 light (after so many usages/recharges, they just eventually don't hold a charge as long anymore, cheaper lights burn out faster than better ones). Get one with a tri-pod mount (most do have a threaded socket in the base of the handle), and set it up over your shooting rest.

I've had lights that mounted to my scope in the past. Yes, it's convenient, since it's pointing exactly where your sights need to see, and the pressure switches that come with them are nice, but they add a lot of weight to your rifle, and make it a LOT more awkward to pack around, not to mention that it always made me nervous putting that much extra strain on the scope body and rings/mounts under recoil.

In my honest opinion, don't skimp on the rifle, don't skimp on the scope, don't skimp on the ammo, and don't skimp on the calls, but a light is a light. A quality rechargeable battery and a high intensity lamp with a red filter and you're gravy. For 300yrds at night, your scope will be the weakest link. A good light gathering glass is critical.