installing WP in a temporary domain

Until now all my installations were new domains and new installations.
I have a running website developed in html.
I want to move it to wordpress.
2 questions:
– how do I setup the new WP installation without moving the domain first and changing the servers? I will rephrase: i need the html site to continue running and in the meantime be able to build the WP site (I know how to do that) and only after the site is ready I want to move the nameservers. I am using hostgator and fantastico, so I am asked for a domain (which I don;t have yet since I did not move it)
– If I use htaccess to redirect using 301 all my pages from the old html to new sites in WP, will this affect my SERPs?
thanks