Long-term sys admin (small amount of hours) Ubuntu VPS needed

I'm running a VPS on which I run about 7 websites: 2 webshops and 1 custom-made webmail solution that I want to expand. Because my current sys-admin is too busy I'm looking for a new sys-admin. The current sys-admin will do the handover. I need 3 monthly checks (latest updates, etc). Someone who is available in case of need: (very important!!!) primarily because I want to expand the custom-made webmail solution and attract new customers a swift response would be nice. Maybe also help with the set-up for new customers.
Security is important. I'm looking for an experienced sys-admin and would prefer to have some references.

Ubuntu, plesk, Apache, Nginx and wordpress sites.