Leo
The chemical sealant will not prevent all punctures but may help to seal the small ones. There are several brands available & most bike shops will carry at least 1 kind. You may better luck installing a Kevlar belt inside the tire between the tube and tire. If you are an experienced cyclist you should already know it's difficult to prevent flat tires no matter where you ride. The roads here in Thailand are strewn with glass, wire, thorns & just about anything you can think of which will puncture an average tire. It's best to be prepared.
Most good mountain and or street bikes here are equipped with 26 inch wheels. I havn't seen any bikes in the Udon Thani area with the new 29 inch wheel as yet. You will also see many fold up bikes for sale which are going to have a much smaller wheel. I don't know how much you want to spend for the bike so it's kind of difficult to tell you what's available. I don't know where you intend to ride so it's also difficult to advise you on the type of bike. There are many dirt roads here in Thailand as well as many paved ones.
My friends name is Peter. He owns a pretty well equipped bicycle shop here in Udon Thani. His telephone number is 086 458-6195 when calling from inside Thailand. I will speak with him about your needs so he will know to expect you.
There is an excellent bicycle shop in Bangkok If you have time. The name of the shop is Pro Bike. Here's their web site:
www.probike.co.th
Please let me know if I can help you guys out with anythingelse.