You have lots of opportunities for collecting data on your clients. Perhaps you could just put your questionnaires on your own web site and ask clients to complete them at the beginning and end of their work with you, rather than mailing a questionnaire. It is a more anonymous way for them to respond. Satisfaction is important and the client satisfaction questionnaire would work well. I would also collect basic demographics and perhaps a short screen like the Mental Health Screen (MHI-5). the items ask on a 6 point scale how much of the time in the last month they
"have been a nervous person"
"felt calm and peaceful"
"felt downhearted and blue"
"been a happy person"
"felt so down in the dumps that nothing can cheer you up"
It works well as a screen for depression, has been validated against the DIS for anxiety and depression as well as a variety of other measures and when I use it, it somewhat sensitive to change over time. Whether you will be measuring outcomes is a matter of debate, but you have to get started somewhere in order to refine and develop it over time.