As mentioned, check to see if they're licensed and insured.
If a quote is really low, they could be the kind of company that takes your money & runs, but they could also be someone trying to get started in the business.
This "could" mean that they do great work, but their price is low to get the work. It could also mean that they're cutting costs by not getting insurance or hiring people under the table.
In addition to the estimates, ask for some references from people they've worked for before. Check the contractor's board for open or closed complaints.
And, get a few more estimates to see where the fall. Some contractors will assume that you have a big budget because you're starting a business and hike their rates.
Usually, the mid-range guy will do a great job at a fair price.
Also, you shouldn't have to pay anything up front, but note that some of the small guys with low prices won't be able to do the job without a deposit because they can't front the materials. If possible, don't pay a dime until the job is completed to your satisfaction.
(I've never opened a coffee shop, but I have past experience in the contracting field.)