You can take pretty much anything and convert it into anything else, so the engine won't really be a big stop for that. Do keep in mind that there is a lot more to models than just the polygons nowadays. For instance, I don't believe that Autocad has a system for constructing bones in a model since it's really not intended to do character modeling.
I'd look into a program like
Milkshape if you seriously wanted to do that sort of stuff. It's designed for low-poly work, can export to numerous game formats, is easy to use, and can be had very cheaply (only $35).
Another thing to consider is the programming side of things. Unreal engine games have their own scripting language which is (arguably) easier to learn than C++, which is required by several modern engines -- including Source, id Tech 4 (Doom 3, Quake 4, etc), and more -- yet is capable of the same degree of game modification. Even the C++ based ones can have some pretty major differences between them, most notably the number of services that the developers provide via libraries, not to mention the level of documentation.
All of that said, Source and Unreal Engine would probably be your best bet. From what I've seem of the Doom 3 SDK it looks to be incredibly well-organized, but it has the disadvantage of having the fewest players.
Kind of a tough call, really. :x
