Do you feel enthusiastic about writing the book? If it's a chore to write it will probably be a chore to read.
If you do, then write it for enjoyment's sake and view any money you might make as a bonus. It sounds like you have potential beta-readers in your interested friends, so that will be useful once you have a first draft.
Have a look at what is selling well in a similar genre. If you are thinking of traditional publishing, have a look at agency websites and individual agents' Twitter to see who might be interested in your book.
If it's non-fiction most agents will look at a proposal rather than requiring a complete book, but producing an attractive proposal is an art in itself. If fiction, you'll need a full manuscript at submission stage although your initial submission will just be the first few chapters.
Whatever you write, there's nothing to stop you self-publishing and seeing what happens, but the more you research self-publishing, the better chance you can give your book of selling. There are lots of blogs and resources from successful self-published authors to give you a start.
Best of luck with the project whatever form it takes!