newbie question - transfering a website

[sjt]

I realize this is a newbie question...please don't laugh too hard...

I have a client that has a rather large website (about 25 pages). It was created in Frontpage...I'm going to transfer it to Wordpress (StudioPress).

I would like to NOT take down the current website if possible...
So I'd like to:
Register a dummy url (www.sjt-demo.com)
Create the website there
Once the website has been approved by the client then I'd just copy all files over to the client's url

Any pitfalls in doing it that way?

Or, would it be better just to bite the bullet, pull an all-nighter and take down the current website?

Thanks,
Stacy