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:
http://www.ovcnet.uoguelph.ca/ClinSt...ts1-14.htm#04.
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.