Any conventional High Speed Steel Twist drill will do fine.
Cast iron is supremely easy to machine. Use no lube whatsoever. An air hose will be helpful. to clear chips.
When machining cast you need no oil or coolant because the chips come out so cleanly and do not adhere to the cutter. This means the heat will stay in the chip. Any lube you use will create an adhesive bond ( surface tension) holding the hot chips against the tool and work causing the heat to transfer to the tool and work.
In a machine shop the only time one will use a lube on cast is when it's possible to literally flood the work flushing the chips away in the stream of coolant fluid.
Otherwise cut it dry.
For a half inch hole I'd drill a pilot hole first with a .250 drill or maybe a number 7 drill: no bigger and certainly no smaller.