I received a quote from a design firm I've used in the past for $8500.

I'd find out which properties this management firm is managing and read tenant reviews online.

After I started my own firm, I realized I needed a mentor who could guide me in business (generally versus real estate specific). 

The seller needed to close quick because they had a 1031 lined up so she held firm and stated she is not buying anything over the appraisal price and the seller accepted to sell at the lower appraised price. 

I went on Thumbtack + asked my agent, vetted about 10 firms.

I agree with MOST of what @Michael Plaks said with the exception of firm size as it should not matter.

Easier to get are jobs with real estate brokerage companies, mortgage brokers, or property management firms.3.

The firms focus is in commercial property and I have learned an incredible amount about navigating city and county regulations, dealing with complicated leases, permit running and expediting and so much more.

Either way, you can't go wrong with him if that is your intent.However -- I'm a bit confused why you're jumping directly to his firm, and seem to believe he's the only one in the country that can add value.  

Around me it is normally the law firms that handle the sales that publish the list, so you may have to make a few calls, find the right person, find the right legal publication, etc to get to the right person who knows what you're talking about.