It doesn't matter at all. You are on a mission, and you have chosen to be on exactly one. Use a gold rule, don't be upset if things goes wrong and make a habbit on constaly expect the best - you should be just fine
You can negotiate abit, but not much. Suggest you to check prices in different places first though, to get the best deal. Also be ready that most motorbikes are not insured. As for cars, most of them as well, though you can find the ones with insurance (strongly recommend you that). Safety: always wear a helmet if you use motorbike, always check you mirrors, try not to go when rainy or after heavy rains, NEVER EVER drink and ride, be very carefull in the night time. Also make sure your fuel gauge works fine. Take care!
Not a big deal at all. You need to have tourist visa, thai mobile phone number and thai address. They will ask you for that. Bangkok bank can refuse, cus they seem to do that alot for farnags, but KASIOKORN or any other should be OK with that. And I suggest you to make a deposit of at least 10-15.000 thb when you do that. Good luck.
Most of them are golddiggers. NEVER make a mistake to marry a bar-girl, lol )) You can meet some nice thai girls mixed with europeans in Bangkok, I think. If you really looking this way, I suggest you to go there and find a girls from a good family, with good education and manners. And YES, they prefer to stay in Thailand, so if you plan to move elsewhere - that's not a good decision at all.