Thai immigration have got stricter recently, and for example you can no longer do a visa run to the border from Krabi. I spoke to the Thai immigration office in Krabi when I was in Thailand a month or so ago, and they seemed to imply that it's better to arrive on a flight, and people arriving at the border by land have more problems than people arriving by air. However, they also seemed reluctant to give a definitive answer on the current rules and how they are being implemented.
For peace of mind I would probably suggest having an onward flight ticket. If you book well in advance, and if you're lucky with the timing, you can sometimes get budget flights for around £20, which isn't much more than the cost of a coach ticket anyway. Airlines worth checking are AirAsia, NokAir and also Lion Air.
Also, immigration can technically ask you to prove you have sufficient funds to support yourself during your time in Thailand, but people have done this by making an ATM withdrawal and then providing the withdrawal receipt for proof. I've never experienced this personally, but at the start of the year there were reports of it happening at certain land borders (which is probably why visa runs to the border got stopped).