I may just build the entire thing outside of EE and then get some pro help in making the EE login and cookies work with it.
Check out CodeIgniter. It’s a PHP framework from the same folks that brought you EE, and there’re a few simple, pretty nice authentication modules for it.
And though I haven’t looked into this, I wouldn’t be surprised if someone has been working on integrating CI and EE.
To the Original Poster, I’d just urge caution when using EE relationships. With the number of products you have it probably won’t be a concern, but we did a tool that had something like 70,000 products and related them to a weblog of company names and addresses, and the exp_relationships table got huge… at one point our total database was 1 gig, and 900 megs of that was the exp_relationships table!
Bottom line is that EE isn’t a data mining tool (nor was it ever meant to be). If you plan on growing, you might want to look into some other tool to run side by side with EE.
Just my $0.02