Well, the engine I'm planning on making is designed towards rendering 3D scene's as opposed to 3D worlds. It's going to eventually be a fighting engine with both 2D and 3D qualities. A good example of this is:
http://img84.exs.cx/img84/2213/capcomvssnk2_1004_screen028.jpgI'm going to write collision detection with 3D objects, but I'm guessing I won't really need it as much as 2D collision.
I've read that 2MB ~ 4MB vertex buffers are a good size to be loading every frame with and you should stick to around 150 polys per batch. Index's are alot classified as "first class citizens" to the video card, so they increase preformance as well.
I still haven't grasped the concept of rendering transparent objects front to back, as far as I was aware; as long as you have a z-buffer it will do a check weither or not it's visible anyhow. The topic of semi-transparent objects confuses me a bit.