The more foolish a person, the more dangerous the motorcycle (scooter). In places like Bangkok, Pattaya, Phuket, the drivers are more dangerous. In the smaller villages and quieter parts of Thailand the scooter is pretty safe, you need to learn how to drive them (don't us your front break in the rain, try to go slow and straight through sand) etc. At 30km/hr its about the same as riding a bike. Its easier to fall of the bike.
I bought my motorcycle (scooter) second hand $1500/US. At the time it was 1 or 2 years old. Its' lasted me 9 years and going strong. I put about $6.00/US in the tank ever couple of weeks. Maybe 4-5 hours riding time. I am not sure but it's super cheap.
I did this for the first six months that I taught. After my debt was paid I stayed on for another 1.5 years. They had some issues with the work permits/visas. The laws changed and the paperwork had to be redone. They already had a shortage of teachers and wanted us to work without proper paperwork and do visa runs on weekends.
All in all, I would do it again, but I would also have a plan B. In the first six months I would recommend finding a direct hire (skip the agency). Most/many agencies pay for only the time you teach so you make no income during long breaks. Schools don't do that. Also, agents are there to make a buck which can lead to problems.
You're considering Phuket? Beautiful place. Transportation will be a hassle. So you will want to live near school or learn to ride a motorcycle. Cars are ok but they go VERY SLOW.
Koh Tao, may not have many teaching jobs but its a great place with few roads so little traffic.
I would be a little concerned $300/mo for 2 bedroom. I pay $500-$600 for a 1000 sqft condo near the ocean (Pattaya). The other gotcha is transportation. You can find some really great deals far from public transportation, so you spend a lot on transportation both money and time.
I am not in a place in life that I can do this, now. But I have thought about doing it in the future. Adopt some kids 10 to 12 years old, feed them, cloth them, educate them then set them free to be productive adults. I am curious how you make out. For me, the advantage would be getting them US citizenship easier, which would ideally lead to better economic opportunity and that is really what I would want to do...
On occasion I have read people's experience, when applying for visas at neighboring embassy, the embassy will issue the visa but tell the person "last one from this embassy" or "last one this year", or similar. Those individuals cross at the land border, apply for visa, and come back. The issue is that they're using the same embassy each time.
Worse case scenario they deny you entry (have plan B, go to Cambodia, take a bus in). Best case scenario (and likely) they let you in no hassle. If the do yell at you (pull you aside) they talk to you then let you in, no warning. You can get a "warning" when applying for a visa.. I have only heard of that from neighboring countries. People go to the same embassy and apply for a new tourist visa, they may provide the visa with a warning, but that isn't applicable in your case.
I think if you have a visa, you are likely over thinking the issue. You're level of concern is appropriate for people who are staying in Thailand indefinitely (like myself).
They mostly just 'talked' to me. The asked "why you come Thailand long time" and I would reply "not long time, many time" :). Then the questions you would expect, why are you here, what are you doing, do you work here.. Then, in both cases, they told me to get a long term visa next time. Key take away "next time" they and I both knew I would be coming back.
I would recommend complementing the country. I love the people are so good, always smile. Very beautiful country, good food, best Thai food. etc. It's not difficult to explain why a person would like Thailand. The concerns are: A) Are you working, taking jobs from Thai people or potential revenue, B) Are you causing trouble. If you have a life in your home country and have a few dollars to spend here in Thailand they want you here.
I have been pulled aside twice in my 9 year history. Never have they asked for money. I would be a little surprised if you got pulled aside. I think you will need to give them a reason to pull you aside (dress like a teenager, or disrespectfully, be rude or discourteous to the IO, etc). I do recommend that you travel with something that shows you will be in your home country (or any other country) in the next few months, school schedule or recent/curry student id, maybe a rental agreement or something. It won't be a requirement but it will help to suggest that you have plans to not be in Thailand.
If you are worried about it, I would encourage you to have a plan B.
Also, please note that your nationality plays a part in the policies. If you are from one place it's one set of policies if you are from another place, another set of policies. Hence, when people from this group (or anywhere else) tell you they've had a certain experience, it may not be applicable to you and your nationality. (in other words seek advice from other Croatians)