Very safe, most dangerous thing here are the roads. High death count, 80%+ occurring on motorbikes. Not much crime, I feel safe walking down any random alley at 3am. So yes, you can walk anywhere without fear. Bad things happen, but it feels much safer here than anywhere in the US.

English is spoken almost everywhere, at least basics. 35m+ people travel to Bangkok every year and the tourist industry here is large.

I spend about $800/mo on rent but I have a very nice condo with gym + pool within 5 minutes walking to the sky train close to the center of downtown. I also do month to month with no contract, if you do a 6 month contract you can find a studio for $200/mo ~15 mins walking to the train - I just prefer convenience and luckily make enough income to cover the costs. Maybe spend another $500-700 a month on food and transportation.

Going out to eat is about the same cost and buying food/cooking and the food delivery services here are great (either free delivery or 50 cents for delivery). Most my meals average $5 usd but I usually eat at higher end place, you can do $1-2/meal if you want but it's lower quality food and ingredients typically. I like to go out, but don't drink often so alcohol isn't a big cost in my budget. Beer is usually $3-5 a bottle at most bars compared to somewhere like Vietnam where everywhere has beer for $1.

I initially went for 1 month (1 week in BKK -> 1 week in chiang mai -> 1 week in Koh Lanta -> back to BKK) and ended up staying for 5 months total the first year while traveling to Cambodia and Vietnam for visa runs. I definitely recommend 3-4 weeks your first time to see if it's for you with a similar route to me to get a feel of the different cities. I liked chiang mai more at first but grew to love Bangkok the most now.

Don't worry about booking much accommodation far in advance as there's much readily available. I just like to book a nice place for my first week to recover from jetlag before I head out and play the rest by ear.

Hope that helps!