You set your bar fairly high my friend.

I doubt that many would jump to make your editor for you. What you ask is a bit of an indie development faux pas. You may end up having to do the work yourself. Especially with the amount of features you are asking for.

Paying is an option that opens doors, but you may have to set a price before you get offers however there may be more 'other language' freelance developers that would be willing at sites like freelancer.com and so on.

What I could recommend to you as a, maybe temporary, solution is to take an existing 2D editor's output data (such as Tile Studio) and create a small converter program (or just do this right in your game engine at load time) which will generate your quad and texture data and save it to a separate file that would be more easily loaded into. You then would have to generate a set of sprites to represent your 3D terrain and heights along with your walls and other such spaces. It might be crude at first, but it'll give you something to work with to test your game engine and gameplay until you have time to get yourself a nicer editor.

If I may ask, why style of visuals are you planning on for your top-down RPG? Are we talking something surreal like '3D Dot Game Heroes' or something more dynamic like a top-down 'Neverwinter Nights'?