The GX680 III fires without film, the shutter will fire, the mirror will rise and fall, but the film back will not wind. There is an IR sensor in the back to sense film, so putting a dummy roll in is useful to check functionality - you do need a roll with film as the IR sensor detects the start of the film from the paper.
I think the III is a better choice than the I or II. The I and II run on a rechargeable Ni-Cad pack or 5 AA's. Most of the Ni-Cad packs are dead or have been/need to be rebuilt, and the AA pack is hard to find. The III runs off currently available Lithium batteries (CR123, CR2s). Both require a proprietary electronic cable release (which you really should have), and the one for the III is hard to find and not compatible with the I & II.
If you plan to use the tilt movements for critical placement of focus plane and not for the "miniature" effect, the Magnifier Hood (sliding loop finder) is very useful.
If you plan to use it in the field, a porter, or a partner that is a weightlifter is also beneficial.