Optoisolation is like insurance. You probably don't need it, but you'll be glad you have it if something goes wrong.

Printers don't use them, but they're not hard to make. Just use one of the circuits in the open source forum.

You do not need current limiting resistors with the xylotex board. It is done electronically. As a matter of fact, you should avoid any board that require them. They're antiques.

Bipolar motors potentially give you 40% more torque for a given motor size, by allowing you to apply more current without overheating. But remember that the xylotex is limited to about 2.5 amps max.

If you buy the xylotex, remember to NEVER disconnect the motors when power is applied. This will likely kill the driver.