If you have to buy a tap, buy a machine tap and not a hand tap. Machine taps are a far better tool. They'll last longer and are harder to break.
If you're going to do this very often I'd suggest getting a second drill chuck. Buy the type that threads on. Get a flat-head bolt to fit the chuck and then drill and counter-sink a piece of plate for that bolt. Attach the chuck to the plate with the flat-head.
Set-up is simple, using just the unfluted part of a dull/dead drill clamp it in either chuck, then clamp it in the other chuck so that they are "nose" to "nose". Lower the quill of the drill press until the plate rests on the drill press' table and clamp the plate to it. Remove the drill shank and you're ready to drill cases. Doesn't matter which chuck you put the case in.
I use the chuck on the drill press to start taps straight, just turn them in by hand.