My wife uses her maiden name and has bought several pieces of property with no problem. I have never been asked to relinquish any ownership I might have and she has never been asked to prove where the money came from.
There's a really important issue here chaps. If a Thai buys land and is married to a foreigner, the foreigner MUST sign the land office declaration that the funds were all her 'personal property' and not part of the 'marital property', irrespective of the following:
a) The marriage has been lawfully registered or not.
b) Whether she has taken your surname or retained her own Thai surname.
c) Whether her Thai ID card says Ms. or Mrs, her surname or your surname.
d) Whether or not the land office official says sign off is required.
If the foreigner does not sign the declaration then the land was acquired, and is being held, illegally by the Thai. Sometimes the land office, or certain officials, only request foreigner sign off where Thai spouse has taken foreigners surname, this is not the correct procedure and there will be no liability to the land office at a later date. No proof of where the funds came from is required.
Where the foreigner did not sign the declaration lawyers say you can go back retrospectively and sign it, but then you are taking a risk by highlighting your situation and penalties may be applied.
However, will anything ever come of it ? very much doubt it unless deliberately reported by someone and it will not affect the Thai selling the land.
SJ