I was wondering what format models are produced in for Empires / Source etc. I do a lot of CAD work (at work, I don't have any software at home).
While you're waiting for a dev (probably Megel) to reply and give you an exact answer, here's a sneak peek into what mod teams generally use for modeling in Source: http://developer.valvesoftware.com/wiki/Category:Modeling
I know DonMegel uses 3dsMax. 3DSMax, Maya, and XSI are the most common tools used for Source, but there are quite a few more programs that can export to source.
Maya has the least support of those 3. If you want to start modelling for Source, I'd suggest either XSI or 3dsmax, both have a bigger community and better support and plugins for Source. If you're going maya, be prepared to get aggravated often and having to deal with lots of shit, not recommended if you have anger issues.
You can import an IGES into XSI, anf you COULDN'T import IGES into Max and Maya I would be both surprised and disappointed.
I've been able to create props and view model weapons for another hl2 mod using maya. Once you do it you get everything setup in a custom toolbar then it's like play with model, a few clicks, and a script compile away to seeing it in game. I'm not saying it's easy to get that first model working but it gets easy. Should be noted that valve has a plugin in the sdk for maya 2008. Now i still have been unable to create animations but others have (with maya). So far my experience is based around decompiling, making changes, and recreating based on existing animations.