Thailand can be a real roll of the dice around that time. IF it's gonna flood then that's the approximate time (plus the first half of October), and I was there some 3 years ago, I think, when papers published pictures of the Ayutthaya monuments in a few feet of water and there was a big attempt to protect Bangkok and the subway at the expense of Laksi, the Don Muang area and points north (whose people really didn't like getting flooded so Bangkok could be saved). It worked, barely. (BTW, if you want to be scared even more, the whole plan depended on couple of dams upcountry holding, even though papers were saying they were old and not built to hold back as much water volume as they were getting at the time. The water releases contributed to industrial areas around Ayutthaya getting flooded out, and that spooked quite a few companies that hadn't factored in the possibility).

Meanwhile, in Pattaya, sections of streets routinely flooded and were hard to pass through on cars and baht buses.

The next year didn't have such a flood threat and even had a few sunny days. You can't be totally sure what you'll get. This year the rains came very late, and there's also the climate-change thing.

So I'd guess you have about a 75-80% chance of things being fine and 20-25% of getting something else. But, let me guess, you got a REALLY GOOD deal to go there.