Hi there
Well, the first thing I would do is view my clip with the QT player using high quality setting. This does normally give me a good indication of how my exported file looks like. So be sure whenever you export a QT movie and viewing it with QT player the High Quality setting is set [command + J]. Now the second and most important thing is the compression. I would recommend to use compressor for your purpose because it has some presets that match your requirements. Select from the presets the one that is more applicable to your client needs and after the compression, view it with your HQ-set QT-Player.
Good Luck.
Hisham