That will depend a lot on the nature of your forum. If the users will not upload a lot of files you will be good with any storage that is included in the Droplet of your choosing.
Now, if you’re going to have a lot of files uploaded then any AWS S3 Clone would be great to get working, I haven’t seen any issue with DigitalOcean Spaces but my use of it is very light so I’d take both @RGJ and @pfaffman opinions into consideration when cooshing the S3 Clone to use.