According to the first post, messages are roughly 30GB and attachments about the same. To take a margin into consideration, double that: 120Gb. This is not a that much...
Also, what is relevant data or useless (only the latest info or stuff from years ago) is something that will differ for each and everyone. For me I look for current data about diodes and so one, but also for old stuff like the PM20 laser console.