We did the NC500 last year in September and the weather was fantastic and not too busy. We went anti clockwise starting in Inverness and did a total of nearly 700 miles. There are lots of nooks and crannies you can choose to detour and whilst SatNav is great, I would recommend that you get the Ordnance Survey map OS Road No 1 - 'Northern Scotland; (Inc Orkney and Shetland - not that you will need this bit !!) see - https://www.ordnancesurvey.co.uk/shop/roadmap-1-northern-scotland.html -£6 . This is at 1:250,000 scale but gives you enough detail to plan your route. I have a marked up map which I am willing to lend you on the basis that you PROMISE to return it to me?
As for accomodation, depends what you want to have as a budget. You will get B&B for £40 a night or good hotels for £120. We did not book in advance because we did not want to be tied to times and destinations but had no trouble getting a bed for the night. We also carried a wee tent - just in case - which we chose to use at Applecross. Scotland allows you to tent anywhere you like (except private garden/home ground) providing you treat the environment with care - 'Right to Roam' - it is a great Scottish thing and most locals will give you a water can/advice. We took 7 days, mostly topless!!
As for 'must see' there are hundreds of sites just depends on how long you want to take and whether yuo are prepared to do a little walking as well. The roads are generally a match for lots of continental routes but obviously shorter. Here are a few which we would recommend.
1. Cromarty to Nigg Ferry it only takes 2 cars but at a push the 'captain will take 3 at a time
2. Castlecraig Quarry road - great views back over Cromarty Firth and maybe some dolphins
4. Dunnet Head - Brilliant views
5. Strathy Point
6. Tomisdale/Skarry loop - fantastic!
7. Bettyhill bay - definitely worth the walk!!
8. Cape Wrath - by boat!!
10. Drumbeg/Balacrhditch loop down to Lochinver
12. Shieldaig/Applcross/Tornap - if you have CB - cos no phone signal!! - this road is a fantastic skite over the moor and down towards Locharron
13. Plo .ckton/Kyle of Lochalsh
I would say that you don't want too big a group or you may annoy the locals but generally we got loads of waves etc and good natured banter with many locals and tourers alke. I hope you enjoy the roads and the countryside - it is different to other spectacular world sites but equally appealing. And to be honest I do not work for the Scottish Tourist Board. A few pics are attached ........ (Apologies for the MG tee hee!!)
Please mail me if I can help you further.