I love this! Congratulations on a smart approach to launching a startup. I got a chance to talk to DHH - founder of Basecamp and hacker. (read about it here: http://blog.unthink.me/what-i-learned-in-1-hour-with-basecamps-founder-david-heinemeier-hansson)

One of his biggest pet peeves is the idea spreading like wildfire that entrepreneurs need to jump head on an idea and let the wind take hold. What he suggest and what is greatly showcased by you here is to hold a job and take your startup as the side business until it can sustain itself off sales alone. Don't give equity left and right because if it does take off the last thing you want is to build something you don't own.

If you have been acquiring customers for your product before it's built - that means that it should be relatively easy (compared to most) to get sales once you launch. You are already doing the bulk of what needs to happen which is marketing - simply boost your focus on getting referrals and case studies off the initial sales, ask clients about their core reason for handing you their money and drive that message/reason home on everything you from website to branded material to PR articles, etc. If you end up hiring a marketing agency to help with the next steps, look for the added value of programmers in staff so that they can coordinate what's needed once things are rolling.
Essentially consider the fact that you need to start marketing it and positioning the product with the right prospect clients, get it launched onto product hunt (i could add it for you) because the truth is that you have 3 months to create a marketing system that is helping you sale.
Don't be too afraid of jumping in full time if you can afford at least a few months solvency for yourself and if you are having sales.