September? Good choice. Things should be quieter and midges less prolific. Do a search here on "NC500" for lots of thoughts about how to do it but I'd sum it all up by suggesting not to be a slave to the main route: have a wander around places like Port Henderson / Badachro, the "wee mad road" to Lochinver, Achmelvich Bay etc. Also consider leaving out the east coast section and maybe combining it with a trip to Orkney in a future adventure.