The main jobs are in Teaching obviously, and you will need a degree in reality to do those jobs these days. Check out ajarn.com, they have some good articles about the pros and cons of applying for a before you enter the country, or after. I think they generally suggest applying after as the staff can be a bit less keen to honour things by email confirmations as may be the norm in, say, a UK office.
There are other jobs, in things like hospitality, but unless you have something that makes you unique over the local people, then that's a non-starter really.
If you get settled and have a address then there are roles with the Embassy of your home country, usually in Bangkok, but you would usually need unique skills or have to be lucky to see the right role at the right time.
Modelling and acting are things that some visitors do, you'll have to have good looks usually. This kind of work is probably a bit few and far between. Marketing roles etc can be found.
Usually the dream jobs with Western businesses based in Bangkok are lined up by the companies with existing staff in their home countries.
If you have any IT skills, then BIG Wednesday are a firm to check out. Again, based in Bangkok.
I'd budget for about 50,000 baht a month, but you can live off 30,000 if you want so long as you don't want to live anywhere fancy. Keeping some emergency savings is a must as you'll be in a foreign country with different rules to your own. Travel insurance to start off with (ie maybe first 2 - 4 months) is a good idea, you can then change to some form of health insurance (for the health/accident side of the cover) such as BUPA because you won't want to get stuck having a huge bill at the hospital. Having said that, I' sure many people just keep a large sum set aside in savings for this event, feeling perhaps that the risk of something happening are minimal. At your age though, I'd be more comfortable getting BUPA health care.
I presume you know what visa you want for a long term stay (if you are planning one of the less sensible options, it may not be best to post about it here). Most people aren't aware of the Elite visa which will get you access to the Kingdom for 5 years for around 拢11,000 GBP. Depends on your financial circumstances, length of stay and how long you will be in country at one time.
In my opinion there is not much point in bringing anything with you except clothes and a good stock of hard to find toiletries because you can't find some things here. Definitely don't bring a car! (Taxes you wouldn't believe).
If you need any medication (for asthma, diabetes etc) just make sure you can find it ok here, most of the time there will be no issues.
