Not sure if it has been mentioned but I would request that all riders have proper health insurance... and then you keep a current policy number on hand (if that is legal???) I just know where I board she has it... that way in case of emergency she has everything ready because more then likely I wont have my wallet with me. Im not sure if that is legal but it doesnt bother me.
Fewer horses are better for appeal of a farm. Like stated make sure you have a set number. More horses will mean more upkeep
I like having an area where I can lock my tack up in as well... it helps prevent my equipment from walking off
Are you going to allow boarders to park their trailer on the grounds?
Just a few passing thoughts