1) We recommend to install ISPConfig exactly as described in the manual. You are then able to use all php options that ISPConfig offers.
2) Suphp does basically the same then suexec, but its better configurable. Only if you want to use fastcgi you should use suexec. but there are no known problems, so suexec with fastcgi should work fine.
3) We might support dovecote in a later release. At the moment, courier is the only supported pop3 and imap daemon and if you configure another daemon that is not supported it always can happen that your setup breaks with the next ispconfig release.