"I've been told that size 400 motors operate at bout 30,000 rpm."

This is a problem which has never been adequately addressed. There are literally HUNDREDS of different "400" motors as there are literally HUNDREDS of "380" and "385" motors. This is due to the fact that that number tells the SIZE of the motor but doesn't tell a thing about how many poles, windings, or size of the wire used in the windings. The difference between 380, 385, and 400 motors is the length of the armature/can. I've personally seen (or have) 385 size motors which turn @5000RPMS at 12 volts all the way up to those that turn 35,000 at 12 volts. The lesser motor draws a whole whopping 100mA (unloaded) while another draws 12+ amps UNLOADED! Look for something rated at most 12,000RPMS at 12 volts.

"do I connect them straight to the propeller, or do I need to 'gear it down? I've hear a 3:1 gearbox mentioned at some point."

3:1 is a good place to start.

Skip Asay