Using the official vB demo setup wouldn't help much in your case. If you're serious about designing vB skins, you should consider purchasing a vB license as a startup investment. Run it locally using a tool like XAMPP or WAMP to speed up the development process, and also set up a live forum to serve as a skins showcase for your potential customers.
That's what I'd do.
That's exactly how I have it set up, with XAMPP. The installation instruction on that are a touch on the vague side, but that's probably only the case because it didn't precisely describe what the forum root is, but in hindsight, should have figured it out right away myself. Other than that, XAMPP is ideal for locally hosting the software on your workstation.