I think even christian would struggle with accommodation at that price in Bangkok over Christmas and New Year, although the OP doesn't actually mention which city he plans to visit. Certainly his list of travel agents looks as if he's thinking of moving around quite a lot.

For a first time visitor, I would suggest staying in Bangkok for most of the time, if you can find affordable accommodation, plus maybe a trip to Pattaya for a few days (since hotels are generally cheaper here).