What I've understood, with the disclaimer that I have less experience of non-fiction, is that it's easier to send in a proposal or synopsis or half-done manuscript when doing non-fiction.
For fiction, you always finish the first draft and edit it several times before sending it to a publisher or agent, if you want them to take you seriously. At least if it's the first contact.
I think the main reason you start with a proposal or synopsis in non-fiction is the prospect to get paid while writing the book, and therefore being able to take a leave of absence from your "day job", etc.
But having a proposal or synopsis bought from a publisher can sometimes cause high-pressure situations.
For instance, I recently read about the process of "The Gospel of the Eels". This book was such a hit, already as a proposal, it was sold to several markets before it was even finished and so the author was working on the book while the translator was screaming for pages and everyone else's tails seem to have been on fire. It seems it worked out pretty well anyway, likely because the author is a journalist and probably quite used to deadlines.
So if the economical situation isn't a problem, and you're not worried someone else might propose and sell the idea, you should be able to finish it before sending it in.
You can always finish it and then propose it if you like.
Of course, finishing the book without a buy-in from the publisher means you're writing it under the same premises of success or failure as any fiction book; It may be rejected... or heavily edited...