>> I have been offered an esc to start with. It has a built in BEC, and is rated a 30 or a 40. Is that a high enough level to handle a rocket motor on a good pack? <<

If it doesn't weigh much more than an ounce, it may be useable. 30-40 amps is more than any Speed 400 can handle. You can run two 6V Graupner Speed 400's off of an ESC in that range. The impedance and switching rate of the speed control determines how it will perform.

Read the Speed Control section (at the minimum) of this FAQ.

Don't start out with the Rocket motors. They are less tolerant of break-in method and you're lucky to get 10 good flights out of one. Start out with the 6 Volt Graupner Speed 400 motors. You usually get at least 30-40 flights on one, and they aren't as easy to hurt.

BTW, don't hook up a 7 cell pack to a motor with no propeller on it. Also keep in mind that these things produce as much power as a 1/2A nitro engine. And also be very careful about not having them restrained properly when you apply power. Unlike a 2 cycle engine, it only takes a bump of the throttle control on the transmitter to go from silent to eating up stuff like fingers.

>> What kind of props are good to start with? <<

It depends on the motor. On a 6V Graupner Speed 400, a Graupner CAM 5 x 5 works well. You can also use the 5.5 x 4.5, but you will probably not go as fast. This depends on the airframe though. If it is a draggy airframe, the 5.5 x 4.5 may actually work better. You can also use the APC 5 x 5.

If you are using a Rocket 4.8V motor, using anything bigger than a 4.7 x 4.7 will burn the motor up, possibly/probably in one flight. The APC seems to be preferred over the Graupner CAM for this motor.

And once again, I'll add one of my own

>> How do I ever figure out what prop, motor and batteries to use?

There is a program called MotoCalc that has a large library of characteristics for props, motors, and batteries. It let's you play with "what if" scenarios. It is extremely useful for determining how far you can push that motor or ESC.