You will need your passport and proof of residence from Immigration. I think car dealers prefer a bankers draft rather than cash as you are talking a lot of money!
I don’t think you have enough money in the UK to get your wife there. I think you have to show you have at least 29,000 GBP rising soon to 38,000 GBP, plus suitable accommodation. Where did you get married?
In order to apply to become a Thai permanent resident, applicants must have had a Thai non-immigrant visa for at least three years prior to the submission of the application and must have three consecutive yearly extensions in order to qualify. The maximum quota is 100 persons per nationality per year