Hi, i currently work in a datacenter as a linux servers administrator, so i believe my skills are quite enough to solve your problem. any kind of linux work is my daily routine.
I did same kind of projects before, please see my reviews to understand my knowledges level.
I think, roundcube is much progressive and usable solution instead of squirrelmail. agree with rest, postfix and dovecot are okay
can be done ini 20-40 minutes, depending of your vps performance