Challenges With Grid Market Theme, Etc.

Using multi-sites to build a site similar in concept to Etsy which will house up to 30,000 member businesses. Have installed Frame Market, MarketPress. Using Grid Market as the theme for the front pages (sitemap pages). The theme is not aesthetically attractive and doesn't give us the appearance we want. Want to use a WP theme from Elegant Themes.

QUESTION: If we deactivate Grid Market in order to use the Elegant Theme;
1. what features that are designed to integrate with MarketPress will we loose?
2. how will the lose of these features impact the ecommerce function of the site?

Time is of the essence.

Thank you,
Linda S. Fitzgerald