Hello,
Follow me step by step :
1. Create for all your characteristics a value "select"
2. Define in your variant product for all your characteristic on "select" value AND define no price to each of this "select" variant
TIPS : if you have lot of products and variant, you can consider the MassAction to reach all required product and define new characteristic value in the same move.
3. Check that the "Display 'add to cart' button for free products" option is set to "No" (HikaShop Configuration, Main tab => Product part).
4. Process an override view in your language file to replace "free" for a notice to invite customer to set their products (
documentation
).
Hope this will help you to achieve your needs
Regards