I agree that there's a lot of bikes that would be great in scotland. The road conditions can be varied though so it depends on the type of riding you want to do.
My first camping trips around scotland were on a cbr 125. This bike isn't the fastest, especially when fully loaded, so I tried to avoid the major A roads when possible. Even at 60mph (close to the limit of that bike on the trip) you'll get cars travelling far too close to you on the A9, because they want you to go faster. Likewise, if you are behind the trucks you'll lack the power to get past safely. I remember a couple of times truck drivers indicating for me to go past them, but being unable to as i was pretty much flat out ! Once I was on the tiny roads it was fine though, but travel on busy routes is sometimes unavoidable.
Overtaking is something you should think about too. There are lots of campervans/caravans/coaches etc in the summer so they can be quite annoying. You might be happy to coast along behind them, but other impatient drivers become annoying as they rarely follow at a safe distance. I'd reccomend something that has enough power to deliver quick acceleration even when loaded.
The other thing you should know about a lot of roads in scotland is that they are single track. The advantage of a bike here is that the quick acceleration and braking means you can get up to speed quickly after passing places, and stop quickly when you see something coming. Progress on these roads can be pretty slow otherwise.
As for off road, I really wouldn't worry about it. As far as I'm aware, there isn't really anywhere that you can do it legally. There are forestry tracks etc, but these are ususally working logging areas and can be busier than you think. I wouldn't risk it.