In Thailand, a sales contract between private individuals, including for real estate such as a house, does have legal value even if it is not drafted by a lawyer. It must be clear, lawful, and signed by both parties. Title deeds are important.
As stated before, the land department will do a contract. But a lawyer can avoidvmissing clauses essential for enforcement (penalties, remedies). or whe it is vague or ambiguous terms may render parts of the contract unenforceable.
On top, due diligence. You should be able to find a lawyer under 20K, maybe 10K. When you buy a property, it is important.