re-installation of marketpress lite

I installed marketpress lite for a client several years ago. All of a sudden, the plugin broke and the shopping cart doesn’t work. When I add an item to the cart and attempt to check out, it reads “Warning: Invalid argument supplied for foreach() in /home/freedom_feeder/ on line 2066Warning: Invalid argument supplied for foreach() in /home/freedom_feeder/ on line 2066”

I was going to reinstall with a fresh download of the marketpress lite plugin hoping it would override any glitches that have occurred. Before doing this, I have 2 concerns:

1) If I delete the plugin and install the new one, all product and purchase history should remain intact since that data is stored in the database, correct? In other words, when I install the fresh copy, all the data ‘should’ be there.

2) To reinstall the plugin, is it best to install over the top of the existing installation? i.e. deactivate, delete existing plugin and browse for the zip file to add the new one?

This is an immediate issue, if at all possible, an urgent reply would be greatly appreciated. Thank you so much for all the continued support!