Confused about what I need for a good web store-pluginwise

I want to set up a sight to sell photos. I have been searching plugins and there sure are a lot of them available. In fact, I have become quite confused. I have some questions...
1) Does the plugin operate totally independant of the theme? In other words does the plugin affect the theme layout?
2) Is it best to find a plugin to create a gallery which I think would act as a store front and then find a seperate plugin to manage sales of these item. I think this would be referred to as a checkout plugin? I am having a hard time finding a well layed out gallery.
3) Or is it better to find a plugin, such as one referred to as a store plugin, and have this one plugin do all the work.
4) I have some pages created already such as a gallery page. But I see that a store plugin will create its own pages such as a product page. So, if I use a store plugin does it create all the pages I would need for my site? The pages I created become irrelavant?