1. Some people wash sprues with a mild detergent in order to remove any mold release agents still on the plastic. I personally have never done this with any of my kit builds.
2. Haven't built either kit but the Tamiya will likely be the easier build since it has rubber-band tracks.
3. No idea, can't help you there.
4. Pre-shading in black is a step some use to acheive shadowing and variation in the basecoat coloring. It's up to you on how you achieve this effect, either with pre-shading, post-shading, washes, etc.
5. If you are doing a 1943 onward Pz III, the correct scheme would be Red Oxide primer with a Dark Yellow basecoat over it.
HTH and good luck with your GB!