Customize Wordpress Multisite User Register

Hello, we need customize the user registration for one of our sites, that use Wordpress Multisite installed in subdomains. (Each user select his subdomain for fre *.[login to view URL]). We like offer a better desing and funcionalities for the users regist process experience, whith a single form that check the subdomain availabity (dinamyc, Ajax...etc) and go to the second step (select template and user data (user name and email). The design should be attractive and simple, on the home page of the site should be the availability checker subdomain (connected to the active sites of the DB wordpress mu) and if the subdomain is free move to the next steps. Order type, Step 1, Step 2, Step 3 ... (no more than 3) will be displayed at each step we have just the site online with wordpress mu and working ([login to view URL])