@djthuma, first up, you can't use Duet wifi and ethernet in the same sentence. They are two distinct products. If you have the boards already then you are (baring major effort) stuck with that board and it's manner of connecting to the world. My preference if I was in your shoes would be ethernet all the way - for cameras and the Duet boards.
I see no reason to deal with the bed heaters separately unless you already have them at hand and even then I would be tempted to somehow change she setup in such a manner that the Duet controls the heater.
This is just my 2 cents of course and hopefully somebody else will offer their opinion as well.
Edit: So let's suppose you already have the heaters on hand and they are independently controlled. Let's further assume that you want to run 100C bed heat. What you would do is set the external controller to lets say 130C. The Duet signal to turn on heat would go to an SSR and the external heat controller would be powered by the output of this SSR. You will obviously need a second thermal sensor to attach to the heat bed and feed that signal to the Duet. You then have a rather interesting safety feature in that the external controller will never let the bed exceed it's set temperature (130C in this case) but the Duet will control the bed temperature in normal operation. Since SSR's can fail in the 'on' mode, this setup gives you a really nice safety net.
Note that I am not familiar with the bed heater that comes with a Modix printer and for all I know it might have multiple sensors and heat zones in which case my ramblings are just ramblings that do not apply.
As always, talk to a qualified person whenever you deal with something that can kill you or burn down your place of business.
