The PART set is worth getting. There is a fret from Eduard for the ZiS-5 that has nice parts to replace all the hardware on the cargo bed so you can build a new one up from HO scale wood. There were several production versions of this truck, and the ones later in the war had a simplified cab and cargo bed.
Jadar ( Poland) have PART and probably a good line on Eduard,, as well as handling the ARMO wheels. Other wheels are available from Hussar (Air Connection, Canada) and Tank Workshop (USA). I like the Hussar wheels best. The Zvezda kit is actually very nice, with a few weak points, but the basis is there for a phenomenal model.
There are several very good walkarounds on the DISH Models site and elsewhere, and several good (Russian) books on the GAZ trucks. Zvezda's GAZ-AA is just as nice, although their GAZ-MM would benefit from new fenders. Lots of fun.
Cheers
Scott Fraser