Rule nr 1 - 99%, you cannot take work that a thai can take/ could have taken..... But offcourse if you have the money , you can buy/ rent a bar, but its not recommended...
GOOGLE : The DTV visa allows stays of up to 180 days with a visa issuance fee of ฿9,304 to ฿40,808 baht, valid for five years. DTV holders can extend their stay once per entry stamp for an additional 180 days with another 1,900 baht fee and may request to change the type of visa within the country