This is about the only exception to the rule of COD (cash on delivery).  Yes, if out of town travel is in the plan, an advance adequate to cover her costs is certainly appropriate.
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Absolutely, if you don't know the guy you should get enough in advance so that, worse case scenario, you break even if he cancels.

First, its impossible to say what a fair amount is unless we know how long he wants to book?  His payment should include time + expenses (limo to and from airport or airport parking, airfare, hotel).

Do a lot of research before committing.  If he is a TER reviewer, read his reviews to get an idea of what activities he likes and whether they are in line with what you are comfortable with.  Also check his references and see if any of them have travelled out to meet him and how their trip went.

Get a deposit that will cover your expenses plus part of your donation.  If you fly out there and he cancels, you are out both your expenses and travel time.  Plus if you are unable to change your return flight, you may have to stay there until your original return time and will lose income from appointments you could not take while you were away).  

Travelling to a client can be lucrative if you hit it off.  He may wish to see you monthly and for longer trips.  Just do your homework and make sure you get enough in advance so that you won't lose money if things fall through.