Upload & Sell: Off
Why go the ZD route at all for your business?
I don't know who your clients are, or what they pay you. But if you've been getting by with a 5D for 30 x 40 family portrait images-- I can't imagine they're super demanding, paying very high prices, etc.
There's a lot of assumption in there, I know.
If it's correct, you'd be just as well off by upgrading to a 5D2, which won't cost you much at all. And you can keep using the lenses you have (unless what you're using is not up to par). Or the 5D3. Or if you need still more, making a jump to 800/E + portrait lens. Any of these options will give you significantly better enlargements out at the current limits of the format.
If you're aiming at the truly high end, with high paying clients, why/how have you kept shooting with a 5D for so long in the first place? And how could a 22mp ZD really live up to the demands of the client?
It seems like a simple proposition to me really: If your clients demand MFDB quality, then they must be paying the sorts of prices that allow you to equip yourself with a relatively recent, 40+ MP back, cam, and lenses. If they're not paying those prices, they have no choice but to be satisfied with a 23-36 MP 35mm FF camera and solid lenses (which will produce high quality large prints). Unless those clients have a local dentist/ortho surgeon/CEO w/ a Phase One hobby and a sell-at-a-loss weekend photographer fantasy.