To apply for non O marriage you could have 400k in Thai bank account on day of application.
That visa would give you a 90 day stamp.
You could enter visa exempt and receive 45 days (from Oct 1) and also obtain a 60 day extension to visit wife (after marriage);
The extension is 1900baht.
BTW you state ....
"You are obviously quite well off, four hundred thousand baht equates to £9567 at current rates"...
As I mentioned it only needs to be in the account on day you obtain a 90 day non O.