Go to Khon Kaen, Buri Ram, Ubon Ratchathani, Yasothon (great little steak house there, hole in the wall), Korat (Nakhon Ratchasima) or even Nakhon Nyoke. They all speak very good Thai and are taught Thai in school and use it most of the time. If you go to the farm villages you'll find them speaking an Isan Laos dialect. Of course you can go West to Kanchanaburi, too. But, your idea is fine.