Hi,
thank you all for you replies.

Your strong warnings have made me quite concerned now - not quite what I was expecting!

The rural places I mention are more places I come across and choose to explore rather than a specific town/village/hamlet or region. I'm thinking of exploring on a whim really; if somewhere seems interesting I might head down the road to find out where it goes, that kind of thing (assuming it's legal and safe). I worry that relying on a bus might mean waiting an hour or two or getting stuck at dead-ends, or having to miss out exploring such places completely.
Plus I can keep supplies in the car in case I get completely lost.

My general idea is to spend a couple of days in Seoul when I arrive and then when leaving the city rent a car. First head to the DMZ to take a tour and then slowly head to the east coast along Highways 3, 43 and 5 (or 56) [via Gimhwa]. Explore the east coast including the National Parks, meandering southwards (no specific route). I'll decide what and where to go from the south once I get there though. Probably avoiding the big cities.

I'm not sure how aggressive you mean by aggressive...is it comparable to anywhere? I've had experience with being on the roads in India, the Middle East and France... (as well as in London).

The drunk driving and "foreign driver is 99% sure to be found guilty" worry me the most though. Insurance would normally cover these accidents financially though wouldn't they???