The proof of funds needed is something like an MT199 or anything similar issued from your bank. For all those unfamiliar with an MT199, anything issued from your bank will do as long as it confirms a balance amount around the value of the amount of BTC you're looking for. In regards to information regarding the transaction such as escrow, names of lawyers and the like, only serious inquiries will be entertained. A proof of funds must be provided before any negotiations take place or any information is provided. There is no incentive to entertain an inquiry from a buyer who cannot commit to a purchase. I hope you understand.
Providing you with a MT199 literally means preparing an irrevocable bank guarantee in your favor. It's basically someone's bank "chatting" with your bank and telling them money is guaranteed available for this transaction. It's a way for a buyer's bank to tell the seller's bank "we are ready to transact".
Why "lawyers and the like"? As I mentioned on my earlier post, private attorneys are not suitable escrow agents for cryptocurrency transactions. Just because they passed the bar and hold admissions credentials for certain states does not mean one can entrust them with 500 BTC or more or that they can be relied upon to send those coins after the SWIFT transmission of funds.
I'm sure you will require MT199 + ID of some kind. These are sensitive personal details. I'm not sure if it would be safe to disclose that kind (and extent) of personal and financial information.
There is no incentive to entertain an inquiry from a buyer who cannot commit to a purchase.
There is also no incentive to commit to a purchase with a seller who cannot provide proof of BTC assets commensurate with the required bank guarantees or similar proof of liquidity. What good is it to provide this sensitive information if negotiations later fail?