You can't drive motors directly from a microcontroller, so the key thing for you to figure out is which motor drivers or motor controllers you want to use. You can then pick your favorite microcontroller to control the drivers (probably any Arduino would work). To select an appropriate driver, you will need to know how much power your motors require (the stall current is a useful spec to know, or at least the maximum current your motors will draw in your application). Pololu has a number of motor controllers that you can connect to a single serial line, which makes them very easy to control from any microcontroller with a UART:http://www.pololu.com/catalog/category/10
Motor controllers are more expensive than motor drivers
, but they offer a lot of advanced functionality (e.g. high-level interfaces, acceleration control, safety controls, support for limit switches, etc) that would be very burdensome to implement yourself on a microcontroller (both in terms of time spent coding and processor usage), especially if you're trying to drive seven motors. You can save money by going with motor drivers, but they take more I/O lines (as many as 21, 7 of which need to be PWM outputs, for 7 motors) and offer a very low-level interface.