The only car rental firm I have used here is Enterprise and I can recommend them. They have a few depots around Edinburgh - www. enterprise.co.uk - you can check their rates and they have a place at the airport.
If you are staying in hotels they all have hair dryers but as others have said things like straighteners will overheat. You should be OK with things like laptops, IPads, phones etc using an adapter.
Good luck with driving over here. Just remember that most roads are 2-way without a central reservation so you need to concentrate, especially at junctions. If you are driving into Edinburgh from the airport the first part of the road is dual carriageway but then deteriorates into a narrow, congested road through Corstorphine. With the advent of trams, driving in Edinburgh is a complete nightmare and something I avoid as much as possible.