The category minimums and maximums module allows you to set the number of products that must be in the basket from designated categories before the customer can complete the checkout. If you don't list categories in the configuration of the module, then those categories have no restrictions. This easy-to-use module lets you terminate the checkout with a store customizable message using store morph code. Include links back to the restricted category and the products in the basket so the customer can adjust quantities in order to resume checkout.
See the module's slide show (link above) for several screen shots which show various features of the module.