This is my personal experience with the buses to Eastern Thailand. The station is in need of a serious renovation but it does it’s job. I guess it will be rebuilt in the near future.
Can get buses, coaches, minivans here. Multiple counters selling tickets to same places so check Google for information about each booth because each will be slightly different. You can just come 30 mins in advance and buy directly, no real need to buy ahead of time. If you want to book in advance you can DO THAT HERE
I went to Huahin from booth 21. Minibus was comfortable had seat belts and AC. Stopped once for short toilet break. Total time 3.5 hours. Booth 28 for tickets to Pattaya seems the ‘official’ ones.
The bus terminal is at Ekkamai BTS.
Phra Khanong, Khlong Toei, Bangkok 10110
NO AC in the station. there are limited stores but a 7/11 is outside. ATM machine available and lockers to store luggage. Some English spoken but its hit and miss.
Bring tissues or toilet paper for halfway bathroom or bathroom in the station