You can get the domain registered from any reputed as 95% hosts will have an online control panel to change nameservers etc.
Right, for a starter website 50-100mb seems good enough

Configuring Google Apps is easy and Google Apps does have instructions on configuring it for most common panel like cPanel, Directadmin, Lxadmin etc
The feature which allows you to have more than one website in one account is either Addon or Parked Domains