I live in Korea and I am an OSM contributor. I see that you probably arrived yesterday, but I hope I can still give you some useful information.
I can't answer your question about using OSM data on your specific GPS, but I guess by now you will have tried it out.
In Korea, most roadsigns (destinations and street names) are written in Korean and English. Some of the instructional signs (warnings or restrictions) are Korean only, but it should be obvious from context what they are trying to tell you. Oh, and the motorways (red/blue shield) are toll roads. The 'national roads' (blue oval) are a high-grade, free alternative in most areas.
Regarding coverage, it's not too bad. OSM Korea is mostly routable, and most of the major roads are present and connected. Some of the smaller roads are not present. Also, as you will see, there is a significant amount of road construction happening all over the country, with some roads being upgraded or re-routed, and new roads being added. It's tricky for OSM volunteers to keep on top of this, but every day the map gets better.
There aren't too many POIs in the database, and some of them may be out of date. Also, Korea changed its street address system a couple of years ago. OSM Korea does not have many old-style or new-style addresses yet, so you may only be able to search using POI name.
If you *do* use OSM data during your trip I'd be very keen to hear your opinion of the coverage and accuracy (good, bad, or mixed), which I can pass on to the Korean OSM community.
Have an enjoyable time in Korea!