Tim,
Nice model! The tyres are well covered in other posts, but one option would be to drybrush with "rubber" colour on edges and the flat running surfaces, but leave some tan on the sides. My reasoning is the tanks arrived in Africa in grey and were resprayed at field depots where they wouldn't bother masking or taking much care to control over-spray. But as soon as the tank was in use the paint would start wearing off the rubber parts. (I remember seeing British armour in Germany being resprayed at the start of the Gulf war - I'm not even sure they let the crew out before the paint started to fly!)
As for when to paint in the build cycle, it depends on the kit. Some details can be retouched after the base coat, while others are best attached after painting. I try to get as much attached as is practical before I paint, but every kit is different.
And the AM tracks all depend on what comes with the kit. I will use the kit tracks if they are decent, but will reach for the AM stuff if the kit tracks are terrible - and only if the kit as a whole justifies the extra cost! Many of the old Tamiya kits are fun to build, but don't justify the cost of any replacement details...
Tom