For a dropdown box just use an ezoption type for your info collection class. If you have a look at the template \design\standard\templates\content\datatype\collect\ezoption.tpl (eZ 3.9) you can see that it displays as a selectbox anyway if you have more than 5 options. Just adapt the code to your needs.
A real combo box (a select box where you can also type in to add a value) requires an ezoption attribute and a string attribute in your info collection class. And then some fancy javascript and CSS to display both attributes as one combobox and some template code which determines which attribute was used. Not really hard to do but still some work.
Claudia
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