How can I find questions from users of a minimum reputation and accept rate?
I don't think there is way to filter questions based on reputation or accept rate. So you're out of luck there. And when it comes to the Data Explorer, if such information could be retrieved and used to filter questions (don't know, never used it), it would be somewhat out of date. So I'm not sure it will be much use when trying to answer questions, unless you don't mind the good chance that there will already be several competing answers.
But given your apparent motivation for wanting to filter on these criteria, I would suggest the following:
Forget about the user asking the question. Disregard his accept rate. Just look at the question. Is it a good question? Do you think you can contribute a valuable answer? If your answer to those last two questions is yes, just contribute your answer.
You're not just contributing information for this single user. The larger goal of Stack Overflow is to build a great resource of information. Not just for the user asking the question, but for the wider audience of the internet.
So you give this great answer and it does not get accepted. And the user apparently never accepts anything. Well, that's a mere 15 points. Nice to have, but if your answer is good enough, the amount of rep you'll gain through upvotes will far outweigh it. Yes, they don't play "the game". And the game is important to the site. But in my opinion there are other things far more important than that game.
So don't let one user stop you from contributing great content which will possibly benefit many more users who will read it.