I also found 'product_kit' to be pretty basic in it's current form.

One maybe easy solution might be to add a node reference cck field to your 'product' content type. Call it, "Related Products", set it to multiple. Then for each product add the appropriate node references to the products that it is related too.

This way when a customer views a product, on the page would be listed the related products as a link.

Then make a views block that would display the related products in the sidebar.

Or make a views block that shows all the related products to what is currently in the shopping cart. Though this is more complicated.