Dom, left field suggestion, what about a Sony A7 (or if you have the money A7R)? I prefer it to the 5D3 and you can use other lenses, I use Canon EF lenses on it, and I have a Zeiss 135mm made for a Contax fit. Think of all those old Leica and Zeiss lens you could get, for portrait shoots.
The down side is it eats batteries.
The reason for going for a Mamiya 645AFD is the fact it's medium format the sensor is twice the size of a DSLR ful frame so the IQ is so much better it's the same as when you look at a 35mm shot compared to a 120 medium format film shot. I if do go for a DSLR it woul;d be the 5diii as you can now pick one up for about £1500 because the 5div will be out soon.