If you want that "rich mellow" guitar sound, rather than the traditional "high silvery" sound, then you want an instrument that is both wide and deep -- the greatest internal volume. Larger internal volumes give more bass/baritone response. Look for a depth of at least 2.5" (6cm) and a body length (not just VSL length) of say 32" (80cm) without the tuning head. You'll probably find the ultimate sound in a Tennessee Music Box or other box shaped dulcimer rather than a teardrop, hourglass or other shape. Type of wood has not been proven to effect the sound of dulcimers to any major degree. Any builder who has made more than a handful of dulcimers will know how much area of soundhole each of his/her designs requires for optimum sound production.
Rather than searching Ebay and similar sites, I would advise you to search the "For Sale" both here and the www.everythingdulcimer.com . You will be dealing with knowledgeable dulcimer players, not someone's great aunt who can't spell dulcimer let alone play one. Prices won't be any higher here or at ED, and the seller won't try to rip you off over the cost of postage.
After nearly 40 years of messing about with dulcimers, here's my partial list of some of the best dulcimer builders in the USA whose instruments show up periodically on Ebay and elsewhere. Most of these builders are still alive.
Bear Mountain (Dwayne Wilder)
Modern Mountain Dulcimers
Bonnie Carol
Blue Lion
Capritaurus
Warren May
Homer Ledford
Gary Sager
Ben Seymore
Mike Clemmer
Jerry Rockwell
Ron Gibson
Jean Ritchie & George Pickow
Cripple Creek (Bud & Donna Ford)
MacSpadden
Folkcraft
Dave "Harpmaker" Lynch
Kevin Messenger
John Knopf
Bobbie Ratliff
Dan C and a couple other here that I've forgotten (sorry guys....)