Hi Bea,
#1. Yes, all your posts will transfer if you install the Market Theme. Also, since it's a theme — if you ever wanted to "un-dp" the changeover, you would simply just re-activate your old theme and you'd be back exactly where you were before.
#2. No changing since you're already setup running WordPress. You would simply upload the Market Theme to your "wp-content/themes/" folder and then the Product Manager plugin to the "wp-content/plugins/" folder. Then, activate the plugin and the theme and you're ready to configure. (full instructions are in the included "readme" file.)
As far as additional method of payment — currently, a bank transfer is not a part of the Market Theme feature list. You'd have to get a PHP coder to modify things to allow you to do that. It's certainly possible, just not part of the base feature set of Market.