Design upsell sequence and integrate with shopping cart

We need someone who knows how to build an upsell process for our website. We have a low priced product that will get the buyers info, we want to be able to do post transactional upsells and downsells to take advantage of the mindseat that the buyer has to increase the value per customer. We also need you to know how to put cross sells in the upsell process: products that are not owned by us, and be able to integrate that into the shopping cart. We have a shopping cart already, but are not sure if it supports what we want, so if you have done a project like this before, and know the best shopping cart we will use that one. This is a time sensitive project that will need to be completed by middle of december. Thanks for your bids