I wouldn't do a spinner for your first attempt to be honest...if you're looking at a design with a moving weapon do a lifter or an electric crusher of some kind, spinners are:

1: Rather dangerous - one of my (now retired) fleaweights was a spinner, and the entire robot weighed 75g with the weapon assembly weighing only about 15g maximum, and it still caused me a few injuries/cuts...any weapon on a robot is dangerous but spinners are even more so and the potential for damaging yourself - even if you work as safely as possible - is still high.

2: More complex and expensive than you might think - a properly balanced spinning mass, some method of gearing down the motor you use, the motor itself, and crucially a method of switching the motor on and off that will handle the current (either a very large relay controlled by a R/C switch, or a fairly hefty speed controller) is going to cost you a fair bit and take up a lot of space inside the robot.

3: Limiting in the events you can go to - you can't run spinners at the vast majority of events that run featherweights, if you had one you'd only be able to play with it once or twice a year at an event with a 2F grade arena (which so far only Robochallenge have, although I believe the RR arena can be set up in a 2F configuration if not for cost)

4: Rather dangerous - safety is always first when building and fighting robots so this point needs making clear! :P

If you've got your heart set on a spinner this can be a bit hard to swallow, and nobody's going to stop you doing one as your first robot if you really really want to, but a lifter or crusher is, in my view, a cheaper and easier (all you really need is the weapon itself, a linear actuator and something like a Team Delta dual ended relay board and you're in business for less than 100-150 pounds and hopefully with all your fingers still attached) way of getting an active weapon for a first time builder.

EDIT: I've just seen the interchangable section which makes a lot of this post look stupid and patronising, very sorry :P but I would still build a non-spinner weapon module first and do the spinner when you've got a bit of experience from that.