Small X-Cart validation Job

We're developing an X-Cart store and need some validation writted to ensure when products are added to the cart the customer cannot order over a certain quantity of products. We have created a field in the database for max products allowed to be ordered. 1. When a product is added to the cart validation needs to make sure they are not adding more than the allowed quantity. An error popup will tell them they need to order less than the maximim allowed. 2. It also need to make sure there is not a quantity of the product already in the cart so what they are adding goes over the allowed maximum. Should be easy for anyone who has worked with X-cart before or knows PHP and SMARTY. Ideally we want someone with experience using X-Cart.