new server

I am going to build a new box (2-3.000 users, 4-500 domains name (50-100 real, remaining used as redirect on the main (.it,.eu,.com,...)) . Is there any how-to or similar to follow for build an updated system? (My current boxes are based on CentoOS 6.x using included Dovecot + PostFix .rpms)

I am thinking to CentOS 7.x + Dovecot + PostFix + RoundCube + Squirrel + procmail with unix users and virtual users. But I admit that systemd isn't still my best friend ... :-)

In CentOS 7.x there is a default dovecot-2.2.10. Is it enough Ok or better use different rpms source?

Any hints is welcomed :-)

Thanks, F.