Hi Hugo,
I'm sorry you got such an abrupt reply from Kerrerra to your first post on the site, stick around and you'll find we're a friendly bunch on this site, and in my experience, you'll usually find several people will try to help whatever the problem.
First things first, if you haven't already done so read the walk report on this site at Walks>Highlands>Loch Ness>the Great Glen Way, and you'll also find more reports on the route in the Walk Reports Scotland forum.
There is no chance whatever of finding cheap accommodation in this area, particularly in August which is the busiest time of year for tourists: in fact I asked the same question on this forum in March/April as I had planned to do the walk in May but prices quoted then were too much for me. As things turned out, I had to defer the walk and I'm doing it later this month, but the relatively few places available at this time of year are still expensive. My wife and I have decided instead to rent a cottage in Fort Augustus for a week, and I'll walk the route in stages by driving my car to the end of each stage, getting the bus from there to the start of the stage, walking back to the car, and driving back to Fort Augustus. It isn't as complicated as it sounds, and it is actually cheaper than B&B for one, which was my original intention. If you are planning to travel to Scotland in your own transport, then you could possibly prebook accommodation for your your group in a hostel or camp site, and walk the route as I plan to do.
As to your group, I'd say go for it, you should be OK as it couldn't be considered remote country, I'd also suggest you contact a youth organisation such as the Scouts (
www.scouts.org.uk/) or the Boys Brigade (
www.boys_brigade.org.uk/) either of which I'm sure would be pleased to offer advice and assistance.
As to the weather, well, it could be anything from -30 to +30 Centigrade, sun, rain, hail, sleet, or snow, and if you're really lucky, all in the same afternoon (I'm only joking, it usually takes two days to get that range of weather).
I hope you and your group manage to make the trip, it's a magnificent part of Scotland and I'm sure you'd all enjoy it