Server setup for multiple wordpress installs thats secure

I have read through the community some older posts for multiple wordpress installs on a single server. Could anyone inform me which environment would be best to setup for that, thats a more current than those articles? I want to setup something pretty quick and move my sites over since my current host is coincidentally closing.

I am also not really familiar with security of a server. I am coming from a managed VPS that I ran sites on previously. But I am not sure what all is needed to keep the server secure. I did read some articles on here that basically say to run a SSH key instead of password. But is that it as far as server vulnerability goes? I know what to do within wordpress files to keep them secure. I have troubleshoot all their software loop holes and have those locked down. I just want to make sure the back side will be locked down as well.

Thank you for any help and suggestions - Clayton