I'm still paying off my medical school loans (30k remaining). No kids, Corolla (paid off), rental apt (split with girlfriend). I have savings ~100k liquid and available to me now (much more is invested) and my girlfriend would support me. My annual expenditure (outside loan payments) is ~40k. Without vacations 30k.
Seems like you've solved the hardest problem up front. If you had a lifestyle close to your income, and close, committed relations to people who were comfortable with that (and only that) then you'd have some serious negotiating and/or losing to do. But since you're already living economically, and have given yourself plenty of room to maneuver, I'd say you've done well.
If your startup were in the medical field (i.e. one you know well, and in which your background gives your insights the halo of serious credibility) then your positioning is even better.
And if you can get to Working Prototype before quitting your current practice entirely, you should be able to make a very solid decision.
In that case, go for it! You have nothing to lose. You'll always be a doctor, but you may never get this opportunity again (low expenses, no kids). If you spend all your time working on Plan B, you'll never get to Plan A.
