get someone who can speak thai to help you. dont go on the offensive.
Get the police involved, ask to see CCTV.
Ask to get a second quotation for the damage from another company, keep your translator with you at all times. Tell them you work here (show work permit) and are aware of the general cost of things. this door might cost 5000 baht maximum.
Forget about the 50k, that is the charge for Mr Holidaymaker