A motor may be more optimally wound for a certain voltage (ie if it says 12-24VDC you may get more torque out of the motor with 24V), but typically you can still run them at slightly lower voltages without any issue. When you start applying more voltage/current than the motor is rated for, that is usually when you run into issues.
The motor I linked to says 12VDC, same style motor drock linked to, his didn't say but I'd be pretty confident that it is also 12VDC, for a much better price than my link!
Keep in mind these motors were designed to run on a boats power system, which is pretty similar to that of a car or truck.
As far as syncing the motors - the Cole Hersee switch I linked to at ryderfleetproducts.com will do this. I haven't checked the price against WH, but without some type of switch or circuit they won't be synced or park correctly. I believe Viperwolf made a diagram of how you could park the wipers without a special switch, but I don't remember if it synced them. The cost of the switch was worth the hassle to me.
I dont have a centech harness, but based on what I've read about them I'd say your wiring kit is more than adequate. Depending on how your fuse block is setup you'll just need to add a fuse and tap into the power somewhere
