Hi there, first, to answer your question 2+5=7
I have many years web development experience and after reading through your document and having a look at the sites listed, I am confident I can work with you to get the solution you require. By the sounds of it, this will be a complex task because each of your product categories will have its own filtering system with different select boxes. Could I ask how many categories there are and what kind of shopping cart system you have in place already? If you could let me know the URL to your site I could play around with it and have a look at some of the categories myself.
Secondly, do you have an idea of how many products on average a category would have? Part of your document mentioned having hidden divs for all the products and displaying those that meet the criteria of the filter, but I'm concerned that may be problematic if there are significant numbers of products in some categories.
Finally, on the demo site you listed, it looks like no matter what combination I select, it only returns one product and a button to immediately add that one to cart. Below that is a table of all the products for that category I assume. Would your site be similar in that there is a table of all the products, and then above that the filtering system would sit, and after selecting the options we'd need a few rows for the varying number of products that match the criteria?
Let me know if I understand everything correctly. All the best, James