I am from UK BUT always send sterling and take the better exchange offered in Thailand. When you select Thai baht you are given a rate as you mentioned, so then as a comparison Google the bank you are using in Thailand and check Telex transfer rate. That's what you get when send $ less admin charge about 350 baht
If you are sure you existing policy covers you for the duration of your stay, it is not necessary to have health cover in Thailand, many choose to self insure instead. The cover against a visa only applies to an OA visa where no funds are required here as the visa is issued in your home country
Suggest you learn some Thai and travel for 1st year. Every province is different, pattaya or Jomtien has bad reputation BUT I don't drink and have great life here. Easy to get involved with sport, golf, sailing, charity work and then make friends with good hearted and business ladies