What bikes are you interested in?
I normally have a pretty good idea of what bikes suit me then try to get a shot on mates/other DHers bikes. my biggest problem is that I like smaller bikes than anyone else I know so most are undersprung for me. Do you know any DH riders? might be a good place to start, get advice etc.
a Demo bike will rarely be set-up to your taste/weight/size/riding style. and even if you find something the right size tyre choice/pressures can make a huge difference to how a DH bike feels never mind suspension settings/spring rates etc. an ex Demo/hire DH bike may also be a bit ropey depending on use/abuse/maintenance