Just in case you didn't already know, there's a kit available for a stepper/servo shield. If your interest is more in doing stuff with the steppers than learning how to design drivers, that will get you running faster.
I don't have a suggestion for the diode array, since I've always used the L293D (which has built-in diodes) for small motor driving. For loads like relays, I just grab whatever 1N4000-series diode is handy. Not as convenient when you need several, of course.
Ran