Installation advice – new to .com

Hope this is in the right section…

I’m a graphic designer and have previously created sites for small clients using wordpress.com with some success. I now want to create a site for a larger client using a custom theme, but am not sure whether I might be getting in over my non-web-developer head…!

I intend to get a psd file marked up by http://www.p2h.com, and have them build it for WordPress. I also intend to opt for their CMS installation and theme activation service, where I understand (?) I just have to supply them with log-in information for the hosting server which will already have wordpress.org installed. OK so far I think.

I’m just not sure what comes next. I obviously need to redirect the client’s existing site to my new one – do I domain map in the same way as I would in wordpress.com? If so, will I encounter the same problems with client’s email, meaning I have to install Google Apps for their email to work..?

I do have some other similar .org/.com questions, but I’ll leave it that for now!
Any advice is much appreciated. Thanks.