Approaching copyright holder for licensing product

There are some health related audio recordings that I would like to offer as digital download. The author is deceased and I will need to approach the widow.

a) What would be a typical-industry standard to offer as royalty %? If I can document that my offer is common, then better chances of being accepted without long negotiation.

b) What software, website or system would I use to assure her of proper accounting and payment? Having a third party handler her payments or at least some reliable system/software would help her to feel comfortable with me. Yes, I can go through clickbank or similar and maybe later. For now, I was just going to sell it myself and pay her a royalty.

Thanks in advance for your intelligent suggestions/experience!