I'm not sure if a Doppler that only took stills would be OK or not - how would you know you were getting a picture of the peak velocity? (There may be a way, I just don't know that much about the details). Here is a site that discusses echocardiography that might help you out:
The Boxer Underground www.boxerunderground.com
has somewhere information on where to purchase Holters - the company they list was selling used ones for $300 - quite a steal! I think the info is on the health-tested Boxers page.
Also, the ABC website has information on Holter testing - I'm not sure if it's where to buy a Holter or just where to rent one and where to send the tapes. http://clubs.akc.org/abc//abc-home.htm
and go to the Health page for the link.