what would be the most appropriate way to archieve something like the template variables of MODX in Wordpress? Is it even possible?
It should be possible to attach custom fields to custom post types and it must have to ability to add different media types (String, Int, Boolean, Image, URL, etc.) to the posts. The entered data would have to be validated and there shouldn't be a need to enter the field names manually (as with WP's regular custom fields.
Check this - http://wordpress.org/extend/plugins/advanced-custom-fields/ and more - http://wordpress.org/extend/plugins/tags/custom-field