Having done a lot of off-season travel for budget reasons, I'd also encourage you to grab this trip.
The one thing in the minus column you don't mention is hours of daylight. EDI of course a fair piece north of ATL. I looked up Jan 19 for you via
www.wunderground.com. 10'14" for ATL and 8'1" for EDI.
Not a reason for not doing the trip though. Just a reason for planning which activities when.
The first time you hear that lovely Scottish burr, you'll know you did the right thing. When a baker hands you an authentic scone in a "wee bag" you'll be even more sure.
I've no idea what your travel plans are, but I do like to hunt down AirBNB places for foreign stays. It really adds to the authentic experience. I can remember all those places at which I stayed (both the good and the occasionally challenging). Standard hotels are pretty much a blur.
Scotland does have a lot of B&Bs also, and I'd take those over hotels. With the car you can reach the more remote ones, and they may well be less expensive also.
I always urge people going to a new destination to go to their local library first and bring home an arm full of travel books. Even novels and historical novels. Anything set in the area. One finds all sorts of little tidbits of information, and the actual trip has more depth. Maybe even buy your favorite to take along, although since this is a budget forum I can admit I've sometimes just taken a library book. The only drawback of that savings is I can't write in it and dog-ear pages. While I usually end up getting Lonely Planet (for the maps and detail), there are lots of other excellent ones.