I agree with Michael. I have always liked Polar. They are accurate and dependable. As Thea suggests, the FT-80 is a nice monitor.
As Susan suggests, finding a heart rate coindent with the onset of hyperventilation (lactate threshold) is a great idea. I like using a treadmill of stationary cycle to determine the HR/threshold Then, do intervals up to that HR, slow down 25-30 bpm. Repeat the intervals for whatever time you work with your client.