The only advice I can give you about money is that if you're likely to use your credit/debit card make sure you inform your bank in plenty of time before you go. Don't take out more cash than you think you'll need each day, leave the remainder in the hotel safe.
Don't know about Koh Samui as I was in Karon on Phuket, but my guess is that you'll find it pretty similar in the following ways:
Be prepared to be pestered to death by the local shopkeepers, stallholders and prostitutes. If you have men in your party then bespoke suits can be made from scratch in three days, along with beautiful tailored shirts. Haggle. Preferably visit three shops and get the best price from each before agreeing to buy. Don't be surprised if you're importuned day and night. Even with me walking beside him my husband was invited to join the 'ladies' for 'fun, sexy time'. One time he had to go out alone at night, (I wasn't able to accompany him to an 11pm suit fitting because my legs were too swollen for me to walk) he arrived home traumatised having been pursued by a gaggle of prostitutes - one of whom was on a scooter!