I would go for a 5mm pitch 16mm or 20mm wide belt. The pinions can simply be drilled and tapped to accept a grub screw to fix it to the shaft or you could use these : http://uk.rs-online.com/web/p/locking-bushes/4888062/ They are a bit expensive but are really good and are easy to adjust and set the pulley on the shaft in the correct place. Personally I dont like attaching the pinion directly to the motor. The shaft can easlit be extended and supported by and extra bearing or two. If you have a look at my build you can see how I extended the Y Axis motor assembly. This might give you some ideas. You can extend the shaft via a coupling or using a solid bar bored out to take both shafts. An idler pulley is simply for tensioning as you say. Mount it on a sliding block with a bolt for setting the tension with a locking nut to secure it once set. What do you mean by how do the bearings work for the idler pulleys?

Ian