Hi rocketand roll. I will answer your question as best i can unlike 'poppva1' who continues to spam this site with pointless comments!
I have visited samui twice in March and april and the weather was fabulous both times, very little rain and even when it does rain it's a downpour for about 1 hour max thensunshine again. Ithink generally December to April are the best times to go.
Ref car hire - There are the usual at the airport like Hertz etc but you could always check this portal for others places locally:
http://www.siamdir.com/auto_and_marine/vehicle_sales_and_rental/index.htmlIm afraid i cant comment on villa hire because ive only ever stayed in Hotels here.
Happy holidays