Maybe be honest with him rather than give him false hope and lead him to more issues. Anyone without money and / or income should be thinking twice about going overseas long term. Common sense.
. Most countries don't require a police clearance to marry. Yours might but he's from UK and doesn't. Shouldn't need a birth certificate either. Passport should be enough for him, and Affirmation that you mentioned.
I think it's the agent taking the piss trying to fleece him for 300 baht.
Or the landlord gets the agent to do tm30 for him and pays him 300 baht not realizing it can now be done online in 5 minutes by anyone. He can do it himself with landlords details and permission.
If she is financially stable and has travelled to other countries without "incident" such as overstating, she wouldn't have much trouble getting a visitor visa. Her application has nothing to do with you unless you are financially sponsoring her.
My wife easily got multiple visitor visas to Aust before and after marriage because she was working full time and had travelled all over the world without incident.All she needed from me was a letter of invitation.
Partner visas are a long , expensive and drawn out process. Not worth it unless you are planning to live there together.