1) I'm not sure what the hang up is. Magento is pretty powerful. Being a one person shop doesn't mean you would need less features in a cart. I don't use free software because it is normally a pain so I just pay for commercial software and then you get their support.
2) I think WP carts are getting pretty good. Professional software is so cheap these days, I always wonder why people starting stores are too cheap to shell out their money. If you don't have enough confidence in yourself to make back the $300 or the $30 a month, you should not be starting a business.
Now, with that stand, if you don't have the money, then go with a free cart until you can afford a paid one. I did that for 6 months. I should have done it from the start. You will probably think the same thing once you start free and go to paid.
3) I instantly start
SEO and PPC from day one. Even when I rank #1 on google, I still spend on PPC. It's marketing, people constantly see your ad and your website name every time they search.
4) I don't do much of that now. I'm not looking to expand with more stores or niches. I'm changing gears a little this year.