What to check before a deposit, what to check before completion, and the specific documents to insist on.
Get a copy of the title deed and have a lawyer run a title search at the Land Office. You are checking that the seller is the registered owner, that the title is a proper one, and that there is no mortgage, lease, servitude or court order registered against it.
For a condominium, ask the juristic person for the foreign quota position, the outstanding common area fee balance on that unit, and the last two years of the building's accounts and AGM minutes. A building with no reserve and a history of unpaid fees is a building with a special levy coming.
For anything off-plan, check the developer's track record of completing projects rather than launching them, look at whether the EIA approval is actually in place, and find out what happens to your money if construction stops. A number of Thai projects have halted at the EIA stage after taking deposits.
Re-run the title search on the day, or as close to it as possible. Encumbrances can be registered between your first search and your transfer.
Inspect the actual unit, not the show unit, and hold a retention if there is a snag list. Once you have transferred, your leverage is gone.
Confirm the inward remittance evidence is in your name and in your hands. The Land Office will ask for it and will not proceed without it.
The agent's recommended lawyer, the developer's in-house legal team and the "free legal service" bundled with a purchase are not acting for you. On a purchase of several million baht, independent Thai legal representation is a rounding error against the price and the only part of the transaction that exists to protect you.