Any Mac in the current lineup should be able to run iAd Producer; so, if you've had your eye on one for a while, treat yourself!
If you've already got a screen, keyboard, and mouse, the Mac Mini is the cheapest way to get started. If you're wanting to start afresh, a MacBook or an iMac will handle iAd production and development just fine.
If you're planning to buy a used machine, any mid-2009 or later model should be pre-installed with Mac OS X Snow Leopard and will be capable of running iAd Producer. Use the steps outlined earlier to see if a potential machine is suitable.