You can add ConditionPathExists= or ConditionPathIsDirectory= to a Unit to check if one or more directories exist. You can also substitute the userid into commands with %U, so you could have it always chown before running the command, as you can have multiple ExecStart= directives.
http://www.freedesktop.org/software/systemd/man/systemd.serv... and http://www.freedesktop.org/software/systemd/man/systemd.unit...