Hi Areonz, welcome to the forums!
First, check out this very disorganized website that I have...lots of Thailand tips crammed in there: http://www.thailandtips.info
Second, money questions are a tricky thing. In theory, you can get by on $30 a month in Thailand if you have your ways.
My best advice would be to plan less time to potentially do more things. You don't want to change continents just to sit in the guesthouse afraid to spend money. For instance, if you can trade spending six months stressed about every little expensive for five months plus try something new and life-changing such as scuba diving...why not go for the later?
$600 a month is pretty comfortable in Thailand assuming that you do not do a lot of shopping or partying. Drinking is sadly the biggest expense for many backpackers.
You can cut that number significantly if you team up with someone - especially in the islands. It may seem strange, but sharing a room with a stranger (either sex) is pretty common and cuts your accommodation by 50% - not bad!
$2000 is very excessive for a flight! I usually fly for between $600 - $700 one-way, sometimes cheaper. Get a cheap flight or make your way to LAX somehow. Sleep in the airport or couchsurf with someone there overnight, then take a DIFFERENT flight - try China Air or Korean Air - from LAX to BKK. Splitting up the flight in LA is far cheaper than going straight from home to Bangkok many times - this is the way I have done it the last three years. Try: http://www.startbackpacking.com/travel/how-to-find-cheap-flights/
1) Yes. I met a German girl, only 19, living on $200 a month! Not easy, but considering that all you need to survive is food and water, anything is possible. I met another backpacker who approached at random an elementary school and offered to teach for two hours a day. In exchange, they gave her free food and a dorm room!
2) Boots are way too heavy. Flip-flops are the norm, but if you plan to do a lot of trekking and climbing, consider getting some sport sandals like Keenes or whatever.
3) You must be out of the country and getting a work permit requires being sponsored by a company or agency in advance...not usually an option unless you sign on as a contracted English teacher. Once your visa starts to expire, you can pop over the border into Laos, Myanmar, Malaysia, or Cambodia depending on which part of Thailand you are in.
I hope this helps!