The answer depends in part on how many pictures etc. you expect your users to upload. We transitioned to Discourse from a homemade forum on top of a bunch of Majordomo mailing lists. On our old platform, uploads were a non-issue. Once we switched to Discourse and people discovered the joys of posting inline pictures, our uploads exploded.
We have about 100k registered users, of which 40k are active. We have 1.9 million messages (about 5 GB worth of text) and about 18 GB in uploads. Those 18 GBs all showed up in the last two years, since we switched.
You can elect to store uploads in S3 on AWS, that’ll save you on local disk space. Same with backups.
Even if you do use S3, if your forum is the same size or larger than ours, I’d go with 80-100 GB of disk. It’s good to have some elbow room, just in case.
Right now, with two backups stored locally, older backups in S3 and all uploads in S3, our server still weighs in at a hefty 54 GB.