The question is about agents, so I’m happy to answer here. I travelled independently, which I recommend from Kanchanaburi. There are also agents who go daily in a minivan from Bangkok and pick up in Kan.
Exactly Tod. There is no other choice. If you want to go there for a visa exempt 30 days, it costs you 960 baht. There is no visa, you are paying for the service obviously.
Build it first without a work permit. If you find you really can sell it for a profit, then set up a company to continue. This is a very tough market to make money in though. House turnover can be very low outside a few hotspots. Great houses at good prices stay on the market for years sometimes.