try supporting an editor like 3d Max or something similar. The flexibility you'd get with something like that is definately worth the cost, I think. It takes a long time to make an editor as full featured as you need it...(more time you can spend on the game instead of the editor)
3D max might end up being to expensive to use, and it's a lot more complicated to learn than something like radient or DEdit. My first recommendation was going to be to support one of those editors, but I'm not sure if their license would all you to use the editor to make something for another game engine.
I've worked with all 3 editors (Radient, Max, DEdit) and I think that if I had my personal choice, 3D max would be the editor I'd choose to use...(I've been using it for a year and still have barely scratched the surface of what you can do with it.)
And considering your claim of not being a 3d modeller, you did a pretty decent job on those buildings. In fact, I must admit that I thought they came straight out of MM6.
Anyway, if you have any technical questions you need answered, I'd be happy to help. I'm a regular lurker on this board, so don't be afraid to ask them here....and I'll answer them here so everyone can share in the answers.