Some book proposals take a long time to find the right publisher. The history of publishing is full of stories of books that were rejected by dozens of publishers, but later became best sellers. As long as your agent has confidence in your proposal and in you, your book has the best chance for success. It's best to have your relationship in writing. After all, this is a business relationship, and affairs that involve money should be spelled out on paper.
Agents can offer one or multi-book contracts.
If your agent doesn't offer contracts, at least get a letter of understanding so that the important terms of your relationship are spelled out. That's the most difficult question to answer. Typical book offers range from zero to one hundred thousand dollars. If the prediction is too low, authors begin to lose faith in the agent's professed knowledge about publishing. The size of the advance may not be the most important element of publishing contracts, anyway.