Wow - you seem like a very strong, very interesting candidate and potentially a very good fit for your target schools. I think you can really benefit from some help shaping your story and marketing it to your schools. Not sure about other firms but we definitely start before first set of essays is written, and can start before or after GMAT. After is fine if you want to really focus on GMAT first and then move on to apps next.
How should one choose an admissions consultant? We believe in getting on the phone, having a conversation and getting a feel for our style. If you like the approach our company takes you can then speak with some consultants to make sure the individual consultant is a great fit for you. No one person is the best match for everyone so you need to screen and see what feels right. We provide free 30 minute phone consultations here: http://www.stacyblackman.com/contact/
Conrad and the Stacy Blackman
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Stacy Blackman | Stacy Blackman Consulting Inc | http://www.StacyBlackman.com | +1 323.934.3936
MBA blogger, US News and Author, The MBA Application Roadmap