I have been taking architectural and RE images for the last 12+ years. Many of my images are used for very large displays and in print publication. After years of chasing ultimate IQ (and NOT going into MF) I have concluded that almost any camera 16MP and above will suffice for the vast bulk of work commissioned.
I currently use 5DmkIIIs with an assortment of Canon TS lenses, zooms and a Rokinon 14 (!).
In my experience the TS lenses are wicked sharp and do a great job of in camera correction. However, in most cases the perspective issue is not severe and can be handled very quickly in post.
What I have found is that the Canon 16-35 f4 L is an amazing tool for FF and offers very high sharpness with the flexibility of the zoom that greatly speeds up shooting.
Speed is always good as many clients (even those who should know better) feel that two hours is plenty to shoot a house and get AD results.
In addition the TS lenses do not have lens profiles despite the fact that they have some small distortions (particularly the 17).If I were interested in spending more money, I would get the Canon 11-24. It has the angle of view needed for the bulk of RE work with enough slop at the wide end for significant perspective adjustment. It also has that 11mm view that will get shots that the client wants that most cannot get without stitching.
I started in digital cameras with the 10D and moved as quickly as possible to larger formats as the available wide angle lenses were few for the APS-C format. Moving to the 5D 13MP was like dying and going to heaven as my wide angle selection was now much larger. IQ improved also but never as dramatically as one might think.
Proper technique, lighting and site prep will make far more difference than almost any camera short of a phone.
Oh yeah, the Rokinon, it is crazy sharp, fast and cheap. It does have a lot of distortion but there are great lens profiles that overcome that and allow angles of view that can save your rear.
Get the 6D it is wonderful and has great IQ. A large range of excellent lenses is available for it and you can grow it into a very nice machine for your business.
I use a CamRanger for my work and it is invaluable and has greatly reduced my post production time through better light placement and details that get overlooked on a small screen. It seems the WiFi of the 6D would be a bonus in that regard.