OP - the goal of being in business for yourself isn't silly. Nor is the idea of using LEGO to do this.
It's just going to be extremely difficult... no, make that impossible ... for anyone here, well-meaning or not so much, to give you the numbers that are going to work for you. Too many factors to consider, too many different ways and mixes of ways to make money selling LEGO.
Under DR's plan (yes, I'm a fan too, we're 7 years debt free now) you can run the business in a small way now, but even at this stage the business needs to be making some kind of profit that is more than you could get taking a second job - otherwise you should be liquidating all the inventory, LEGO and otherwise, to get you through steps 2 and 3, and then returning to the goal of starting a small business at that point.
DR's base 20% is a perfectly reasonable place to start - If you are aiming at $60K/year small business income and have a family's worth of deductions and tax credits, that 20% is likely going to be enough to cover your tax burden, and you will of course sit down with an accountant at some point and determine if your quarterly estimates need to go higher than that.
Most folks here aren't trying to do LEGO as a real business. A few are, and some number more are using it as a long term investment vehicle, which doesn't put them in a useful place for offering info to a small business owner.
Sitting down with a DR-approved accountant at this stage might be very beneficial -- they can give you some good starting advice, and you can determine whether you should be trying to do this at all in steps 2 or 3, or if it's best to wait until after.
My humble opinion. I'm not among the nay-sayers who think selling toys for a living is ludicrous. I'm doing it now, and did it with books before that and training dogs before that, so being an entrepreneur doesn't seem to me to be a silly or dangerous goal.