Just back from a week in Scotland - we had a wonderful time and beautiful weather - and I will write an official trip report next - but wanted to call out some specifics about car rentals since that gave me the greatest anxiety when planning.
We took the train from EDI to Inverness and planned to drive to Dornoch (an hour north) for two nights, then to Plockton (near the Isle of Skye) for two nights (so east coast to west coast).
We rented from Thrifty Car Rental in Inverness and they were awesome - I highly recommend them! We arrived on the train and you call them from a pay phone at the station (the bank of payphones are straight ahead on the wall of the station when you first get off the train). They answered the phone promptly and were at the train station to pick us up in 10 minutes. They drove over using the car we rented, so we only had to pack the bags once, and drove us back to the Thrifty agency (5 mins) and finished processing the paperwork, showed us how to use the GPS, pointed out all the scratches on the car, etc. We were off in absolutely no time and on our way.
Now for more specifics, I reserved a large sedan for 4 adults plus luggage - and they upgraded us to a beautiful brand new Land Rover Freelander - this car was SO nice. I took the additional insurance for 12GBP per day to avoid a $500 deductible- and for peace of mind I was glad I paid it b/c as a new car, I worried anytime it was parked in a car park that someone was going to open the door into the side - and the "scratches" and stone chips they originally listed as damage, I think would have buffed out and wouldn't call it damage at all - so I figured better safe than sorry by paying the additional fee.
For driving in the Highlands I could not imagine driving it in a sedan. We took a lot of single track roads, over hill and dale, mountains, etc and as someone that drives a small convertible, I could not imagine taking my small car over a lot of those back roads. If you are planning to drive the smaller roads, definitely go with a 4WD vehicle - and bring your Tums!
Returning the car was just as easy, we drove into the lot and the person at the desk took the keys, hopped in and drove us straight to the train station.
Thrifty Car Rental was so fast, polite, efficient and well priced than I would recommend them to all.