Well, I don’t know exactly how to say this but if you’re going to be undertaking a project as complicated as an e-commerce site, having to learn WordPress menus is going to be a very small and very uncomplicated part of that. In the time it took you to make these posts and do some research on free themes with the integration you probably could have already figured it out and had a top nav menu with your categories in it.
That said, themes aren’t really the kind of things you want to skimp on. The best software costs money. WP e-commerce is great and all, but really is pretty meh w/o goldcart, which costs money. If you want to truly rock and roll with menus (w/o crazy development time), you need ubermenu, which costs money. High level, rock solid themes: guess what? Cost money.
If you want to make money/a strong product, you probably are going to have to spend a little bit. Compare that with hiring a developer to build this all from scratch and WordPress costs basically nothing.
As far as e-commerce themes, everyone seems to love stuff from splashing pixels, but I haven’t found their support to be very good for issues beyond basic, and I do mean basic, administrator menu settings. They do work though.
