To be honest, I'd be looking at getting a dedicated modeller using a proper 3D package, like 3DSmax, Lightwave, etc. and actually understanding the model format he will be exporting for in the final engine.

All full 3D packages have exporters, and what you want is a good pipeline for assembling blocks into levels, (which you could do in MED).

Get him to experiment a bit - collada, fbx, obj formats may all help get textured models across, and between the team you should be able to agree on specifications for models/artwork and full levels before really starting to commit to any engine and 3d pipeline.

Don
have a great day