Hi Christina,
Teaching whilst studying for a PhD is good experience but it does have a huge impact on the time that you can spend on the PhD, not just in terms of hours available but being able to switch from teaching mode to PhD. If the 840 hours are actual contact time (i.e.preparation, marking etc is additional) I would strongly suggest that if you can afford not to teach focus on completing the PhD in the shortest possible time. You gain teaching experience after the PhD is completed.