I don't think you can do exactly what you want. Unfortunately you can't automatically select a target unless it is in your party or targeted by a member of your party or so on down through the focus/target chain originating at a member of your party.

you can use target, targettarget focus, focustarget focustargettarget etc.

you could use the target=mouseover if you have mouse control of the priest

regarding the renew, you could use

Code:
/cast [target=focustarget, help, nodead] Renew; Renew
The first condition to be matched is cast so if the focustarget is harm or dead then the renew after the semi-colon will be cast on the priest's target or the priest if the target is nohelp or dead