Somehow, I never got back to this thread. In case anyone needs to have the info still, here goes!For downloadable and emailed books you don't need to have an ISBN unless you want to list your books at ebook stores, etc.
For CDs and floppies, if you want to list them in Bowkers Books In Print---where book sellers go to look for new title pr on Amazon.com or any other place of the sort, an ISBN and a barcode is needed. The ISBN number identifies the publisher of the work and is a requirement when using distributors and bookstores, etc. They get testy if you don't have one. The bar code lists your ISBN and the book's price which gets scanned in the stores.
However, if you're just selling off your site and don't ever plan to deal with any sort of store or distributor, no ISBN is needed.
No ISBN is ever needed if you're going to be approaching an agent or publisher with your book, anyway. As I mentioned, the ISBN identifies the publisher---not the author---of the ebook or any book. So if you just want to be the author you don't have to worry about anything except writing the best book you can. Make sure you send in clean copy, no typos or spelling errors; check your grammar, proper, or, at least, reasonably proper punctuation.
I wanted to be an independent publisher of my own stuff, so I bought my block of 10 numbers. I have a publisher---for what I do with my writing partner since that's what he wants to do. However our publisher isn't one of the big ones, so we have to do a lot of the promo work ourselves. Which is what I do for my own stuff, except that I have more control over my work. And the profits are all mine . . . less the fees from the places processing the payments for me. Still if the stories sell well, it'll be more money than I'd get from royalties from a traditionally published book.
As Sunil said, start small if you want to do it yourself. Baby steps. I don't have a ton of money either. And since each binding or format of a book is supposed to get its own number, this alone can get expensive. I may kill the floppies entirely, leave numbers off downloadables, and give the HTML and PDF CDs and the print editions the ISBNs---when the cash shows up for print editions.
For those of us in the fiction sector, ebooks are still viewed as "no fun." I've elected to allow printing of my novels in hopes that will attract some readers so if they want to curl up in a chair or in bed, they can.
I also buy ebooks as well as trying to sell them. My fiction library has around 10 or 12 titles. Maybe more, I really haven't counted them. One day when I have the time, I want to add a review page to my site for fiction titles. (Family themes, no steamy stuff.) And I plan to keep adding novels to my electronic library. The stories themselves are good, and some are exceptional. Be sad to pass up work like that.
Anyway, I hope you have had success with your novel, and I wish I'd remembered I'd posted to this thread. Guess I didn't check the email notification at the time. 
Neenah