I'm hoping to achieve success by creating the "Mint.com" for accounting for start-ups.

My question: Should I start coding away? I can understand making a minimal viable product for something "chancy", but I feel like an accounting package would be a sure thing if completed, and marketed right - for almost any niche.

I wrote a complete dating site (ericanight.com) from start to completion, and it seemed to have failed- so I want to insure I don't waste another 6 months.