Kuala Lumpur is 24 hours but you have to have insurance, police check from your own country and a medical in KL. Phnom Penh had fewer requirements in May, but they take a week.
You definitely want a car on Samui, taxis are extortionate. If you're thinking of buying a car/bike, you will need to buy 'certificate of residence' from immigration. Also need one for your driving licence. Probably easier in your wife's name. If you want a 4x4, buy a car in Bangkok and drive it or get it shipped. We live on mainland opposite Samui and I spent a while waiting for a few year old 4x4, ended up buying new, everything was in Bangkok, nothing down here. First class insurance on a 1 year old Mitsubishi Pajero 4x4 is around 20,000 baht a year.