I've driven rental cars in many different countries.
Some left hand drive, some right hand drive and I've also driven right hand drive cars in countries where they drive on the right rather than the left.....hmm....that sounds a bit confusing.

However, I've never had a problem after I've driven for maybe a day or so.
Biggest nightmares have been in Los Angeles and getting off the Peripherique in Paris.

The UK (as is the case in a number of other countries), manual transmission is more popular than automatic, although I think automatics are getting more popular. If you are renting, the costs for an automatic will normally be more expensive that renting a manual transmission vehicle of the same/similar type/model/size.

However when in countries or cities which I am not used to and don't know my way around, I would rather pay the extra for an automatic as it's one less thing to have to do!

The rental car companies also have different types of insurance cover on offer. As mentioned in a previous thread, I normally use payonarrival.com which has been taken over by worldwide reservations but if hiring the vehicle for more than 13 days, they gave you free insurance for a second driver and certain other coverage. I can't recall what they all were but it was a good deal.

In the past I have also taken out annual insurance for rental cars. There are some companies which specialise in this. Haven't done it for maybe four or five years but it used to cost 50 GBP per year. It basically insured the insurance!
It provided coverage for driving a rental car for up to one month on any one single trip.

I don't know if this is the same thing or not, as I haven't looked into it.....but this might give you an idea:-

http://www.insurance4carhire.com/

As for driving in Scotland, once you are outside the cities, driving is easy. The one track roads with passing places can take a bit of getting used to and if you have time limitations.....be careful as a journey can take longer than you might think it will!

Parking in Edinburgh, especially in the summer months can be a right pain. However, I've normally managed to get a space in the multi-storey car park at the bottom of Castle Terrace.

Driving in the centre of the city can also be a right pain....for a while even the locals were unsure of the road system one day to the next due to the construction works for the tram. Each time I return to Edinburgh the road system/markings around the west end/George Street/Charlotte Square etc have changed.