You should know that this is primarily a user-to-user forum. Dreamhost does have employees who occasionally chime in, but you should use the pre-sales contact form to contact Dreamhost directly.
As far as I know, there's no demo web panel. However, the Dreamhost wiki has a number of pages covering every aspect of the web panel. They're all listed from this wiki page:
Queries are measured but not limited based on a set limit. If your use impacts other users, you will be asked to move to a dedicated server or VPS plan.
They do not. They do limit the number of concurrent open connections and again, they will notify you if your site affects other users on your server.
Use the [color=#CC0000]3DOM50[/color] promo code for 3 extra lifetime domains and $50 off
More Dreamhost coupons here!