Almost any tent will do for the Highlands because, when the chips are down, you can, and will, head for a bothy or B&B. The bothy scene is less healthy than it was back in the days when I first visited Scotland, but the remoter shelters are still excellent.
The one attribute a tent for Scotland in summer must possess is serious midge-proofing. I found the circular door on a Mountain Hardwear tent to be the best because I could get into the tent without letting any of the nasty, little beasties in with me.
If some do get in, lie on your back. Most of the midges will go to the highest part of the tent, where you can squash them with gentle pressure. Then you can deal with the two or three that were bright enough to settle near your feet.
Also desirable is room for living in (and cooking in) during steady rain. Try to avoid inners with sloping doors as condensation will fall in even if rain doesn’t.
Many people criticise tents which pitch inner first. I have pitched such tents many times in the rain and never had much of a problem. Get the fly over as soon as you can and have a cloth for wiping out the inner.
When Scottish weather turns wild, it can be very nasty, but the thing which will probably catch out a first time visitor is mild, muggy weather. You really will not need much of a sleeping bag for a mid-summer visit and a fleece might be used only for a pillow in a fortnight of hard backpacking.
Winter is a very different kettle of fish.