You should come on a long stay visa such as an O, OA, OX, LTR, or Elite. If you enter on a tourist visa or DTV you will probably not be able to open an account.
I also have an LTR and am living in Bangkok, Phra Khanong. Congratulations on your LTR.
I budgeted 150,000฿/mo and I really can’t spend that much but I am living well . . . and I have a girlfriend but she isn’t much of an expense. I agree with a previous comment, $3,000/ mo would provide a nice lifestyle.
There really is no answer to the cost of living question but Bangkok can be pricier.
I am tracking every baht this year. I recommend the extra effort to do this