I've started or helped start a couple of small businesses. Most of them tanked 
and the experience might not be 100% relevant to you as none of them was in the US but some basic principles are the same.
First, keep in mind that you don't own a business, a business owns you. You've got to be comfortable with that.
I'd support P71 comment that getting a loan to start a business is not a good idea in general; it'll be the millstone around your neck in bad times, plus you might have to personally guarantee it so if the business fails (I'm not saying it will, just plan for the worst case scenario) you'll be sitting their for years paying it off. My RX7 specialist was in that boat, he was looking at losing his house because he had to personally guarantee loans for the business.
Get a good accountant, preferably someone who already looks after other business in the same sector. I would still learn how to do the basic accounts myself if only to check over what the accountant produces, but let the accountant deal with the minutiae of the whole thing. A good accountant will also be plugged into a network and can recommend other professionals if you might need them - for example, my accountant was able to refer me to a specialist lawyer during a recent contract renegotiation. I wouldn't have found that specialist myself and he proved invaluable.
Check if your employment contract contains any non-compete or "you're not allowed to run a side business" clauses. If it doesn't, I would try to get the business up and running on a smaller scale as a side business. That should be possible with a mail order business; it's not easy to do this rather than veg out in front of the TV in the evening but it'll show if you can run a small successful business, plus more importantly it will show you if this is something you actually want to do without you losing your shirt.
As to the business plan, unless you have to convince your bank manager to give you a lot of money to start one, I'd skip the bit about income projections and other financials. I have yet to see a projection that survives initial contact with reality. In fact, most plans don't - the idea of writing one is to get your thoughts together as to what you want to do. It won't survive long-term contact with reality either, but it gives you a starting point. You know where you are, you know where you want to go, so you can then write a todo list. Oh, and IMHO a good business plan will fit on a single letter page, especially if you're not playing management BS bingo.
One rule I've always followed religiously is to pay your suppliers and the professionals you engage within a couple of days of receiving their invoices (obviously unless the invoice needs talking about). A lot of small businesses don't and max out the payment terms - if you have a reputation as someone who promptly pays their bills, this can and will earn you a large amount of goodwill if and when you need it.
