Awesome. I have only gotten custom suits in PEK, but the sense I get from friends is that the workmanship in BKK is a little better. My big regret is that I didn't get more shirts!
Here's an adaptation of suit buying tips I've sent to others traveling to PEK:

Suits:
Ask for "working buttons" on the cuffs of your jacket - it's one mark of a custom or higher end suit.

Shirts: Their default is to build in the collar stays (at least in PEK), but they'll make them with slots for those removable plastic stays that are more common on our dress shirts, if you'd like. For cufflinks, they will do whatever you'd like, french cuffs (for cufflinks) or button. Personally, I wouldn't get more than a couple shirts with cufflinks - generally, cufflink shirts should be worn under a jacket, and there are lots of settings where they may be too formal.

Bargaining (don't really know how this works in BKK, this was my China advice): Just start bargaining far below your price goals, smile a lot and be nice as well as tough, and walk away if you need to. They also want to get you out of the shop as quickly as possible after you've paid. So: -Take pictures of what you've ordered, and write it on your receipt (since their notes will be in Chinese)

-Be nitpicky at your refitting, and when you pickup. Even if you don't try on everything, which you should, open and inspect everything. When you find loose or missing buttons or seams, they can fix them in a couple of minutes.

Lining: I'm not totally sure about this one. But it may help to tell them what type of climate you live in/what seasons you plan on wearing the suit. That will also help with picking the fabric.