The following is not meant as a legal opinion, but merely to help inform the prospective buyer. Contact an experienced and reputable attorney for full details on current law.
As of 2011, non-Thai nationals are excluded from purchase of land. However, 100% ownership of buildings on land is not a problem. Current laws allow for 30-year lease of land with two 30-year extensions. This may be increased to 60 years + 60 year extensions.
It is a simple matter to have a lease agreement and then enter a lien against the land so that it effectively cannot be sold from underneath the lease. Thai contract law is clear and well-understood.
For land ownership, a company has to have 51% Thai ownership and voting rights, and the Thai partners have to demonstrate financial means for the ownership.