I have ordered a panzer III with a set of metal tracks from Hobby Havoc. I tried contacting the guy from RC Tigerland but did not receive any reply after a week.

When ordering from Hobby Havoc people should know that there's a hidden cost which is the GST. That is a bad practice and I think they should include that in the price shown. Anyway, the total sum is about the same.

Now the next part is to read about what to do with my tank: is it breaking-in the gearbox as mentioned in the FAQ?

One question about most tanks: why are the batteries all Ni-Cad but not NiMh or LiIon? Any reason other than cost? If the battery pack is 7.2v can we use 2 of the 18650 LiIon cells to make the pack? Would the size be the same and would there be enough room to fit if the 2x18650 is bigger?