You can rent a house from anywhere from 5k through to 100k a month, but if you want something mid-tier nice, you can get a newer furnished house
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x2 in a nice estate with a pool and community facilities for around 25k first or second ring roads. After that, the cost is relative to what you want to eat - if you eat Western foods in restaurants all the time, the cost is more than eating a mix a Thai and foreign. 50-75k/month, all up for everything, would be comfortable.
you're describing the very center, not the entire city. And op is asking about renting a house which means outside the center - good luck riding a bicycle from San Sai or Hang Dong
Not sure whether you will be asked, but if youve been in Columbia in the last six months, you legally need proof of Yellow Fever vacination to enter Thailand along with what Tod said.
You can be self-employed and get the DTV, the problem in your case is that you need to show evidence of that and while it may vary between embassies/consulates, things like tax returns, previous evidence of payments and contracts can/are asked for as evidence.