I am not sure how to purchase a scooter. However, I know that some people do buy bikes/scooters to travel around.
Most people - like me - rent out scooters. Almost every tourist town will offer rental. Pai is a popular place and probably the cheapest. I remember it was 150b for one day for non-automatic, automatic 200b. Petrol will add another 150-200b / day. The road from Pai to Mae Hong Song and up to the Chinese village is very-very beautiful. And indeed a bit dangerous, not because of traffic or police, but it is windy and steep. So, please drive very carefully. From Pai - you can also do side trips, however if you want to go to forest, mountain roads, you will need to rent an enduro. The soil is very sandy and super-steep.
In any case, be very-very careful.
I did meet/see a lot of foreigners with injuries from riding a scooter/bike. Actually, quite a lot. Most accidents happen, because they are drunk, ride too fast, not used to local traffic or they underestimate steep and windy roads.
I cycled around Thailand with bicycle - in my opinion Thailand is very safe country for cycling and Thai drivers are very polite and careful with cyclists (save that for Bangkok). The single biggest problem was dogs. It was a constant headache - especially in very small rural roads. So, always watch out for dogs. But usually they don't attack you, they just chase you. I would get off my bicycle and walk slowly away until i was out of their territory.
Nicest places to ride: is more-or-less along the burmese and lao borders. The whole of south (mainland) and the rice plains in the middle of north are actually quite boring after a few days. Try to buy a map and just follow that. You will always bump into foreigners, and have fun but you can also enjoy the hospitality and friendliness of thai people at the same time. Thailand is a mix of backpacker-life and a bit of adventure. Very nice country.
Have a safe and nice trip!