There are too many variations here.... Are the quad-plexes all on the same level, or multi-storied?...naturally, stacking them takes less ground space than leaving them on the same level, but... if you are doing this for 'senior citizens' then you really should consider leaving them on ground level, and easy to get into (no steps, wide doors, handicap accessible, etc -- this all takes extra space)
First check with your local zoning ordinances.... you will find that many areas will not allow multi-family dwellings. You can appeal for a zoning change, if you find a piece of property you really like, but it may take some time.
Also, you should check with an architectural firm, and then a general contractor, and a reputable real estate firm.....they can give you some ideas, and help 'fill in the blanks'. This sounds like a big project, and you don't want any last minute surprises. ;)
Good Luck!