I wrote a non-fiction book based upon experiences of our battalion in War in Israel in Jenin (took place 3 years ago.) My agent took my very basic outline and got a publisher and senior editor interested. They immediately asked for an OUTLINE - in this case a day by day one, (and sometimes hour by hour), which made sense.
No one asked me to finish the book before they would consider it and it was stated (not just understood) that the contract was based on the Outline and on the first 100 pages.
On the other hand I am off and on working on a cook book for single, divorced and real busy parents, called, "Help! I Have A Fire In My Kitchen" and the publisher who is only "somewhat" interested so far, wants the whole damn thing.
What they all do want to know, and I think any major publisher or agent will want to know is as follows:
1. Why are YOU the best to write on such a subject?
2. What unique traits do you bring to the table (as it were)?
3. How long will it take you to complete? (The real final MS. which will be in the editor's hands and going through revisions.)
4. What you can do to Market the book. - Don't turn your nose up at this one. Threads here are devoted to it, and the Internet has hundreds of sites warning writers that they MUST market their own work as well. (It is a long haul.)
5. Your other writing credits (no matter how large or small)
(I am sure there are other questions but these always stand out in my mind.)
As for writing on spec. I always thought the rule of thumb (and rules, as we know, are made to be broken), is that Non-Fiction is sold based on a very comprehensive Outline and a few chapters, but fiction usually requires the entire ms. for authors starting out. I have heard my agent tell me that is all BS and it depends on the Publisher and the agent and the author. (The Holy Trinity). Some will demand a complete non-fiction ms. and some will settle for a fiction outline. All is on a case by case basis from what I have seen and have been led to believe by reputable sources.
Final advice is to try and go for the agent, unless of course you have someone like Nomads, a small independent publisher working with you and your own team. And even in that case I would try and get an agent, even if you already have a contract. Good agents nurture writers cause they are in it for the long haul. And you may have another 30 books in you.
My own two cents on the matter
Teddy