The type of custom fields you can set are as follows:
- Single-Line Text Box
- Multiple-Line Text Box
- Single-Selection Menu
- Single-Selection Radio Buttons
- Multiple-Selection Menu
- Multiple-Selection Checkbox
A 'Single-Selection Menu' custom field has been added to the online
demo if you want to take a peek to see how it can look.
You can set a default value for custom fields, but there is no fill-in option for something not provided in the drop down menu. You could add a 'Single-Line Text Box' in that case, but it won't get added to the drop down menu.
The search page does allow for search by category, but there is no modification available to search by custom field, though custom fields are searched automatically by default as are title and description.