Your initail startup costs are going to depend mostly on your inventory.
If you know PHP and HTML you can put together your store with only labor and maybe a few hundred if you choose to buy a shopping cart rather than use a free one. For a few hundred, too, you wouldn't have to know PHP ( or even HTML but I'd think it would be handy) and you could hire someone to do that for you.
Then there's shipping supplies, some kind of shipping system, storage supplies and any software you might need. And advertising. Aside from inventory you don't really have much initial outlay in money, but quite an investment in labor. Your ongoing costs are advertising, shipping supplies, hosting fees and inventory, plus storage if your product takes up more space then you currently have.
My business has just started to turn a profit after about 4 months. My product involves huge inventory costs... I'm about $11k in the hole now and expect to get down to almost $20k before turning around for good. If my sales go as I expect them to, with only my business to support me, I should be out of debt in 5 years. This is all based on projections that are educated guesses at best and assume very frugal living.
Is the store full or part time? Huh, I don't know. All I do is take care of ds and work (posting on mdc is "work" it's advertising ;-) ) so I guess it's somewhere in between.
I read "Entrepreneural Parents" or something like that, I got it from the library. It was inspiring, if not really "how-to" helpful. To learn html and php, try www.w3schools.com
for some great free tutorials.