Well, I am jealous and envious of this great opportunity! 50 Apartments - oh boy! Marketing this kind of community must start with cultivating relationships with your city, senior services agencies, rehabilitation and medical facilities, and places where active seniors are out enjoying their lives. Sometimes, I think people picture Seniors as end of life when for many, it's the beginning of fulfilling their dreams.
I would start with visiting other senior focused communities and making sure my community's amenities and resident activities will appeal to the person I hope to recruit. For example, will there be a common area for movies (theater), a dining area where gourmet meals are served, a common area for crafts and educational activities, an outdoor gardening area for flowers and vegetables, are pets allowed, an infinity pool indoors and a fitness center? I would build my campaign around these interests and attack it by marketing to the community where Senior work, play and already live. I would also contact new home builders of Villa Communities (maintenance free living) and see if anyone building might need short term housing while their villa is being built. That might be an option to get you through by allowing a unit to be designated for short term housing.
Obviously, I think you will want to make sure you have a presence on the web as Seniors are surfing, too. I would look for brand ambassadors, too - maybe a current resident who is very outgoing and involved and can help spread the message about your community. Good luck!