Last year, I had a rental from Krabi and went to Samui for few days. No problem at all, good roads, big island. We had economy car, no issues. Two ferry companies from Surat Thani, with service each hour or so. Can take train down south and then rent a car. Can combine with other places. There are many options, it's all up to you.

As for airlines, forget Expedia. Look at AirAsia, Bangkok Air, Nok Air.