I was wondering if it is possible to download the materials .i.e users and roles, courses and course content etc and use those to create a similar demo site of my own with my own domain name in a short time?
It'll be better if you download windows installer package, and modify it.
no server setups, no internet required - Demo Ready as when you want it - will need slight modification that you ideally should do.
else demo site is always available, show your customers capabilities of moodle there.
2 cents on demo sites ...
IF you've provided 'sample course' that previews what's possible that's one thing.
IF you provide them 'template demo courses' for each, then while your intention is good, you've also set up a situation for them to learn how to reverse engineer - which is more difficult.
If you provide them a blank course, they must build .. which am assuming is the way production server would be run - unless profs enlist teaching assistants to do the course and then they will be in for a surprise when TA moves on and Prof is now 'in charge' again.
If you provide a template course that has minimal institutional resources in blocks or some such then they build the content and choose the tools to be used.
If demo server and they retain access after training, good place to try out new themes new plugins etc. ... new approaches to teaching. But, demo servers need to be maintained just like production servers ... adds to server admins responsibilities.
So it's kinda wide open ... but can tell you from experience ... any direction chosen won't please everyone! Danged if you do ... danged if you don't. :|