Technically, the word Koh means island.
Assume that you knew that. Number one thing to do is to check, check, and double check that your insirance on the hire car is first rate. Second thing to know is that at some point you will be stopped by the police; just pay up for whatever 'law' they claim you have broken. Third, are you really going to leave a car parked somewhere whilst you get to Samui. Fourth, driving is not for the fainthearted although you will have so much more freedom if you pluck up courage and do it. They do drive on the left, which helps, although often left means almost anything.

To save on cash, try getting the train to koh samui.

http://www.seat61.com/Thailand.htm

A great experience

No problem to drive to Samui. But if you plan to rush it then you'll miss out on some nice sights. On the way you could stay in quiet resorts on beaches that most foreign visitors never see. Ban krut for example south of Hau Hin or those around Chumpon.

If you have the car then a day or two detour to Khao Sok National Park is definitely worthwhile - again not many foreign tourists, mostly Thais but some amazing views and raft houses to stay on. Excellent kayaking too.

When you reach the ferry pier in Suratthani for Samui, no need to leave your car on the mainland - just take it on the car ferry like everyone else does.

When the police pull you over and at some point they probably will - usually for driving in the right hand lane of a dual carriageway, just smile and try to plead ignorance. If that doesn't work hand over 100 baht when they ask to see your license and you'll be on your way immediately. Think of it as an on the spot fine but with no points on your license and no messy paperwork involved.

just save on your hotel budget... the price range is wide