Simple SSL

• Get an SSL certificate (can't do that for you, sorry.)
• Activate this plugin
• Enable SSL with one click Always backup before you go!

• The plugin handles most issues that WordPress has with SSL, like when you're behind a reverse proxy/loadbalancer, or when no headers are passed which WordPress can use to detect SSL.
• All incoming requests are redirected to https. Default with an internal WordPress redirect, but you can also enable a .htaccess redirect.
• The siteurl and homeurl are changed to https.
• Your insecure content is fixed by replacing all http:// URL's with https://, except hyperlinks to other domains. Dynamically, so no database changes are made (except for the siteurl and homeurl).

On a site where the source consists of about 60.000 characters, the delay caused by the mixed content fixer is about 0.00188 seconds. If this is too much for you, fix the mixed content manually and deactivate it in the settings.

Yes. There is a dedicated network settings page where you can switch between network activated SSL and per page SSL. In the dedicated pro for multisite plugin, you can override all site settings for SSL on the network level, and can activate and deactivate SSL in the network menu for each site.