purchasing an e-comm template

hi people! i’m very new to wordpress. actually i haven’t installed it yet. before i do i’d like to clarify a couple of questions i have.

i currently have an online retail shop (which i did myself using magento and it’s really buggy) so i’m thinking of changing to wordpress when i saw a really nice theme that i wanted to buy.

my questions are:

1. do i need to install a wordpress shopping cart plugin for the retail shop theme?

2. if so, does it have to be compatible with the theme or just any cart plugin will do?

3. how long can my theme last? what i mean is i customized my prev magento site, and now with the updates in magento and browsers, my whole site/theme is screwed up. if i actually purchase a theme, how long will i be able to use it before it gets outdated?

would really appreciate some help. thanks in advance!