Glad you're enjoying Ubercart. Regarding the things you're looking to change, some can simply be changed through settings forms. I'd look through the product and checkout settings fields, and you should find where to rename the buttons and zone to state. Regarding the removal of the optional message, this will most simply be done using the locale module (which I'm not familiar with), but may better be done with a simple custom module that removes that description from the form.

Restricting the number of products in the shopping cart is a little more difficult. You'll need a custom module to control when products may or may not be added and to disable the quantity boxes. This is a known limitation of the 1.0 version.