source sounds.gcf has Half-Life 2's sounds (in hl2\sounds\).
episode 1 shared.gcf has Episode One's sounds (in episodic\sounds\).
episode two content.gcf has Episode Two's sounds (in ep2\sounds\).
I hope they're right about it not being much harder; that gameplay video makes it looks pretty intense.
In any case, the new additions look like a lot of fun.
I think you can get the .obj file to compile without any conversions, but I'm not 100% sure.
To be sure, you'd need a modelling program with an .smd exporter (3DS Max, Softimage XSI and maybe Maya).