I thought I had best report back in case it helps anyone else in the future.

Thanks for all your advice it was appreciated although a few things turned out to be better than expected.

After arriving in ROK, within a few days I did decide to rent a car and escape Seoul. I managed to find a very good 2007 English/Korea roadmap in the Youngpoong Book Store (Near to National Visitor Centre in Downtown Seoul) along with a pocket dictionary/phrasebook.

When I picked up my car, the Avis agents offered me a GPS system. They programmed this to the English setting for me as well as give me an English manual and this was for no charge!

I spent 5 - 10 minutes getting used to the car and setting the GPS before heading onto the roads (which I was very glad I did before setting off) and although getting used to the directions given by the GPS took a little while (at first I did miss a few turns and highway exits) I eventually did get out to where I was going.
Traffic was very busy when I picked up the car but it didn't feel impossible to navigate and other drivers knew their vehicle dimensions well so although they got close, we never touched. ^

Once well out of the city the roads were practically empty. Getting around here was easy often because there were only one or two roads to choose so didn't really need to follow place names. And when I was driving in the north there were plenty of military checkpoints that prevented me from going down the wrong road. I think I could have managed these with just my road atlas and a compass, but the city areas would have been impossible without the GPS.

The GPS system warned me about speed cameras (as did the Avis agents) so I think I avoided these fines. The only big problem I was worried about was once when the police waved their orange batons up and down at a T-junction - I think I sped off when I was supposed to stop!

Petrol (Gas) Stations were all either Petrol or LPG and attendant served so filling up the car was actually easy. I just had to tell them to fill it all the way up or add KRW 20,000 worth (showing two fingers for 20,000 worked) so it was very straight forward compared to what self-service would have been like. Quite how much fuel I was buying I never knew though it did feel a tad expensive.

A few of the places I visited were only accessible by car so I definitely wouldn't have seen them without it. Similarly a lot of the farm roads and temples I explored would have been to out of the way to get to by public transport or at least not served regularly enough to visit in a day.

I only ever used the Expressways twice. The first time I picked the right time of day and avoided the jams. The second time was on the way back to Seoul and after a lot of queuing I exited (first exit after joining) and used the back roads. Not only were these back roads free but also quicker as there was a lot less traffic and no queuing.

I would say however that the design of the roads and the manner of parked cars or other obstructions blocking lanes without warning did mean I had to drive with a view to expecting them all the times.

Getting back into the centre of Seoul to return the car wasn't a huge problem except the routing the GPS gave - it involved a few U-turns and getting to the entrance to my accommodation was hard (I knew the area but not the address).
Finding my rental office to drop the car off was also hard - so a good tip for the future is to save the rental location in the GPS when the car is first picked up. It was right next to a cross roads and I couldn't tell which road it was on so I ending up getting help at the local police station.

Almost ready to return the car, somewhat surprised but very happy that I had avoided any problems during the past 2 weeks, a moped driver parked their bike behind my car whilst I was going to the station.

The moped driver parked with about half an inch to spare (quite why they did this I don't know) and then proceeded to drop the bike onto my car! With a scratch and dent to my paintwork I tried to get annoyed with the rider but I just couldn't manage to for some reason. I never exchanged details but I did take the licence number.
Fortunately the damage wasn't spotted when I returned the car (the car did come with a lot of scratches and small dents anyway) so I still walked away with a smile.

All in all, I had a great time and would do it again. Not as hard as you might expect but still a worry if something does go wrong. And I did see a lot of people jumping red lights so you really do have to be careful!

Editted To ADD:
I forgot to mention...the police often drive around with their lights flashing and can be a little concerning. However, this doesn't mean they're pulling you over or trying to get passed; it just seems to be about being more noticed when they are driving about generally.
I learnt that it's only when their sirens are sounded that you have to move out the way!