Hi,
In recent times I flirt with the TV3D 3D engine. I searched the web for all 3D engines with a cost up to 300€ and I concluded that the TV3D programming framework suits to my style..
Meanwhile I use a very stable engine but it is based on DirectX7.0.
I have built with this engine a general purpose level editor (XP) for 3D games and I think it can help you because it focuses on entity placement and not on DirectX9.0 capabilities
Its name is Unlimited Drop Pro.

Here is my web site:
http://moraldigames.com/You can find details on how to load UDPro files using an SDK like TV3D. The process is really very simple.
I think of building the next major version with the TV3D SDK after its purchase.
I hope that you will like it.
Don't hesitate to contact me if you have any questions
BTW: Does someone know how to redirect the TV3D output to a specific window? (under XP)