You've got to decide on an overall style and then match your creation of assets to that style. For instance if you are doing blue print maps, id suggest using a vector program to make really stripped down overhead drawing of furniture (though there has to be a lot of stuff available online for that already I'd think). To create photo "realistic" top downs that aren't photos I'd have to say 3d is the only way. You can take either orthographic or perspective renders as well in such a program, or set it up to take perfect isographic shots too.