Magento extension - Buy in multiples (products by carton)

We need a packaged Magento extension that can be easily installed, just like a regular extension, with the following functionality: a) We sell different types of products and some of them must be sold in multiples. For example Widgets A, B and C must be sold in a box of 6. The box can contain any combination, for example 3 of A, 2 of B and 1 of C... or all 6 of C. Then Widgets X, Y and Z must be sold in multiples of 12 - agin in any combination of them as long as they sum 12. Finally, there are many other products that can be purchased indicidually. b) The extension should let us create any number of "types". Each "type" being comprise of a name (e.g. "Rectangular Box 12x" ) and a number of items (e.g. 12) c) Then products could be assigned a type. c.1) Products without a type can be purchased individually. d) On the front end, customers can shop like normal but Magento should not let them proceed to checkout if they have, for example, items of type "Rectangular Box 12x" that do not add 12, 24, 36 etc items. The shopping cart should validate that all "types" conform to their multiples before letting the customer proceed. A custom message would alert the user which specific products do not comply (e.g. "The following items must be purchased in multiples of 12. Please update your quantity to the nearest multiple") , so they can proceed to add more items. The extension should automatically create the required Attributes or MySQL tables, our client needs to install it by just FTP and no tweaking through phpmyAdmin or any other technical requirement. To win this project, we expect you to: a) Demonstrate previous experience developing Magento's extensions, not just themes or minor $30 projects. b) Express an understanding of the requirements. c) Once being awarded the project but before starting programming, we will need some static HTML wireframes demonstrating how the app will work, to make sure we are on the same page. d) The extension MUST be developed using Magento's best practices. Meaning no SQL or PHP hacks. Only Magento API etc.