What digital camera system should I upgrade to? Professional photographers only?

I am a 15 yr. in the biz experienced film photographer. I shoot a Bronica gs1 6x7, a 4x5 Toyo view camera, and mess around with a Nikon d90. I will be stepping in to the fashion arena and need to make an investment in going digital. I have been looking at the Hasselblad 503cwd with a cfv back16mp and a Mamiya RZ67 proIID with a zd back 22mp. If you are a professional photographer, let me know your opinion, what do you shoot, am I missing anything, would a 645 work the same as the RZ67. Hate to spend over $10,000 and regret it

I think ideally for you the Canon 1DS MKIII would be the camera. At 21MP, it is unequaled. With the work that you are doing, the 35mm lenses would be a major plus and Canon L lenses are superb. As we all know, 35mm lenses are sharper and much faster than medium format lenses. Just think, you'll be able to throw your backgrounds out of focus with lenses as fast as 1.2. Canon has a 50mm 1.2 and an 85mm L 1.2 they both are a photographer's dream come true. Unlike the medium format, you have a very wide variety of zoom lenses to choose from, many of them 2.8 or faster. They also have a number of pimes that are 2.0 or faster. With this system, you can also move into the mystical world of fisheye photography economically. Their awesome 15mm 2.8 is less than $600.00 which is in great contrast to the cost of a medium format fisheye lens and it is sharper too. This camera can be had for $7999.00 and with 2 or 3 of their fine lens for about $10000.00. I say go for it!

For $7000 how can you beat the flexibility and the affordable lenses of the EOS-1 Mark III ds? I understand that is it not medium format...but it is darn close. Excellent buy for the money.

Check out phase one digital backs, they make them for a variety of medium and large format cameras.
I am not sure about your Bronica but I did see a reference to Toyo large format units.

http://www.phaseone.com/

My first inclination is to tell you to go medium format digital. Hasselblad, Mamiya and Rolleioflex all make digital backs now. Most of the fashion photographers I've seen are running medium format.

If thats is not something you want to do or is cost prohibitive, and want to do DSLR, then my only 2 recommendations to you are Nikon D3 or Canon 1Ds-Mark3.
Plenty of power, options, and flexibility in either one.