Is it possible to limit items ordered & or display an error?

I have downloaded v1.5.1.2 but before I start I have a question.

My client wants to limit the total number of items a customer can purchase. He wants them to only be able to buy 7 things. Some products are combinations (2 or 3 things). Is there a way to add a custom variable that says how many items a product is (1, 2 or 3) and either

1. not allow a user to add more than 7 things to the shopping cart (and display an error or message on the page saying they have exceeded the amount they can buy)
or
2. display an error message at checkout if the user has ordered more than 7 things.

The things are different products, but he still wants them to be limited to 7 total. Any thoughts?

Thank you in advance

This commercial mod might meet your requirements:
http://theqdomain.com/ocstore/oc_modules/minmaxorder

Thank you very much for recommending that module.

It did work, but it would have been a nice touch if it checked the items in the cart before the final step of checkout.