The midges have not been bad this year, so far. I use this site
http://www.midgeforecast.co.uk/ to get an idea of what the wee bu**ers will be up to. The Smidge stuff they advertise on the site seems to work quite well, at least it does for me.
Weather wise you should be OK until the end of August/mid September. I have had some amazing weather up north late in the summer and into autum, this year has been wet though...and of course investments can go down as well as up!
On the trip I covered the whole of the North Coast 500 route (anti-clockwise) except the section from Loch Carron to Inverness, from Loch Carron I headed for Fort William.
Going north to Inverness and the start of the route, try and avoid the A9. At Perth take the A93 and head for Braemar, then the A939 for The Lecht, Tomintoul and Nairn then hang a left for Inverness - far better roads than the A9.
There are loads of campsites all along this route - have a look at
http://www.scottishcampingguide.com/ for sites and information. I never booked any camping and never struggled for a pitch and remember you can wild camp in Scotland if you prefer that to sites or can't get a pitch on a site.
Most of all, take your time and enjoy it! On my last day I pressed on from Ullapool to home (I live in Linlithgow, almost bang in the middle between Edinburgh and Glasgow). I should have stopped over around Fort William and spent more time enjoying the section from Ullapool to Fort William.
For general Scottish info have a look here -
http://www.undiscoveredscotland.co.uk/
Enjoy
I should not drink 
and be in charge of the computer at the same time!
