I usualy allow about 100 USD per day, but the real answer is not so easy. In Bangkok, beers can run 60 to 120 Baht or so in most places, but you could pay a little more at high end bars. A nice meal for 2 can be found for 600 to 1000 Baht, but you can eat cheaper, and also pay more. It also depends on how much you drink with a meal. Taxis, entrance fees, Skytrain etc. are reasonable or even better than reasonable. I rarely have a cab fare over 100 Baht, accept to the airport. Food and drink is usualy cheaper outside Bangkok.
To my knowledge, there is no real benefit to Sterling or Dollar traveler checks. I do not take checks, but simply use my ATM card. I have seen other posters say that travelers checks offered a slightly better rate, but I like the easy access of ATMs. In short, I do not think it really matters, and I would not pay any extra fee to get dollar checks over Sterling.