Yes , you can extend. Some immigration offices will do it on every entry, some will only do it once. I would go and ask.
You would need your passport, copy marriage cert, copy wifes i.d card, copy house book registration of your wife,(if applicable also a copy of condo/house lease, your landlords i.d copy, landlords house book registration where your staying.
Its strange that they never put what amount is required as proof of funds. I tried asking before but never got an answer. Id go with a bank statement with over 20k baht as that is the proof of funds they ask for at the airport. But who knows.