Where to stay in Thailand is a big question and difficult to answer for a wide range of different people. Generally you should have been in Thailand on holiday before and know at least one area. Before you settle down however you should travel a little around and look what Thailand has to offer.
Most Expats settle down near some tourist location in the south, like Pattaya, Phuket or Koh Samui, because you can simply buy everything you need (even complicated German Food) and most important, you have a lot of other foreigners to talk to and make some friends.
Still, some people have a dream and move with their Thai (girl)friends in the deepest province in Isaan to set up a farm or just build a house and live in the middle of a Thai community.
Others again want to trade and export cheep handicrafts, so they look for a place in the North like Chiang Mai or so.
Where to stay & What to do
You have to know by yourself what you want but make sure you look around first to compare the places before you decide. All places have their advantages and disadvantages and all places will have surprises for you, which you would never think about.
Phuket is the most international place for expats. While in most other places foreigners are mostly either German, Swiss or from scandinavian countries, Phuket covers the whole range with expats from Australia, New Zealand, the UK, US and most european countries.