In which order should one build up a multilingual site?
I would suggest starting with setting up the multilingual backend. Besides what Drupal core already offers you probably want to use the i18n module. Then you could move on to content migration. If there is no module that can do that for you I'd suggest writing a PHP script rather than manually copy/paste-ing (seeing as the amount of data seems to be fairly large). You could make use of either node_save() or drupal_form_submit() for this task. The first option is a quicker way of adding nodes while the second mimics more closely normal node creation by invoking additional hooks. There's a solid amount of information about using them to create nodes programmatically available on the web that you can make use of. At what point to work on appearance (templates and theming) shouldn't matter too much, but it's probably somewhat easier at later stages when you already have the content to work with.