Hello, treeman82 --
Congratulations on the immense responsibility you have come across!
The short answer is: Look your client(s) in the eye with the most controlled, steadiest gaze you can muster and thank them for the opportunity to work with them and the trees on their property. Impart to them the understanding that there are no "quick answers", all decisions have far reaching effects. Dead tree/wood removal may be considered cut-n-dried for the sake of argument (pun intended and appropriate). Still, everything they do from planning to pruning to planting will have effects far beyond their years.
For the long-term, treeman82, develop this as a relationship not unlike any other long-term relationship -- say your vocation or marriage -- not to be taken lightly -- but requiring all the integrity you can offer in terms of full, well-developed answers to your clients' questions and concerns. All with the understanding that you bill for your time, not unlike your clients' auto mechanic or dentist or restauranteur or florist or attorney. And follow through with prompt, thorough answers and counsel which will allow you to stand tall in the company of those of us who are your peers.
Step out there, treeman82, with the honesty and integrity expected by YOU and your client(s) and your equals. If you offer a relationship rather than a not-to-exceed-$$-proposal, if your client is ready for that, then you are providing what your horse farm owner needs. You might need to be the kind of salesman who can provide a proposal now which links to a later open ended contract. The basis for the whole thing is the honesty of your interaction with your client.
Hey, you probably didn't ask for a philosophy lesson, treeman82, but that's my $0.02 worth. I'm sure others on this Site will jump in with their words of wisdom. If you have more specific questions, ask!