Any dropshipper worth your business should be able to provide a list of their products, descriptions, pricing and skus (at the minimum) in a standard spreadsheet or csv format. This can be used to handle import into a system like Zen-Cart with some modification of the Easy Populate Add-on.

If you do not have a spreadsheet program then I recommend getting one. The best is Microsoft Excel. OpenOffice provides a distant runner up but for me fails in the useability category. Every product that I sell is in an Excel Spreadsheet with macros that allow me to print pricelists for every category or single categories. Each category is in its own worksheet within the spreadsheet itself. I can easily update my prices across a category if need be (as in a recent 10% increase of prices across the board). I currently carry over 1,000 products in 10 categories.

I am currently using Zen-Cart as well but will probably migrate back to OSCommerce after the release of Milestone 3. The coding is just better in that new version. The coding and optimization in Zen-Cart is pretty atrocious so I wouldn't recommend it for a large scale retail site. The developer's don't know anything about optimizing a MySQL query and have queries within loops making it very resource heavy. Unfortunately, it has some features not available in OSCommerce. Zen-Cart is actually based on OSC though, the developers simply changed the names of the functions, added some features and released it as their own project. The name changing breaks backwards compatibility with OSC which is a shame because there are hundreds of modules and contributions they could have used otherwise.

Never really evaluated any of the paid solutions. When I started, they were too expensive and way out of the budget. My online store (which is a mess marketing wise currently) started as a favor to a friend who owned a local retail business. Over time, she made me a partner in the business because she didn't have the time to devote to it due to family and health issues. Back in 2003, the retail store closed and I bought the remaining inventory.