I am in a similar position, though I would like to approach this issue a bit differently.

The most time-demanding (and monotonous) task is the addition of all the product data, at least with a site that will contain 300+ items. So I am wondering - Can I, and how could I best, develop my content so as to leverage my product data for use with a future, 6.x release of Ubercart?

I'm referring mostly to custom content-types via CCK, and perhaps categories as well. Are there any recommendations for my pursuing a site development path like this? Is it possible to do without a crazy amount of headaches when integrating Ubercart into my 6.x site containing perhaps hundreds of nodes on a custom content type?