Are you sure you want to take on the already firmly established software companies that have already amassed hundreds of millions of dollars such as Turbo-Tax for self-preparers, or Lacert for the pros?

You'll need the ability to write software or have to pay big-bucks to those who can, and you'll need a ton of money for marketing against the already established companies.

I don't like to dash peoples dreams but sometimes discretion is the better part of valor. You could conceivably spend millions of dollars and never have a finished product.
Good luck,
Denny