My question would be, why do you need a more powerful server? Are you monitoring your load and seeing it's overloaded often? Are you just looking to be able to hook more drives to it? Do you need to re-encode video on the fly for other devices? Giving some more details would help someone to give a more insightful answer. I personally am using a Raspberry Pi 4, Chromebox w/ an i7, an old HP rack server, and an old desktop PC for my self hosting needs, as this is cheaper than buying all new hardware (though the electricity bill isn't the greatest haha, but oh well). If you are just looking for more storage, using the USB 3.0 slots on the Raspberry Pi 4b you can add a couple extra SSDs using a NVMe to USB 3.0 enclosure. For most purposes the speeds will be fine for most applications.
As for SSD vs HDD, SSD hands down. The only reason you'd pick an HDD is if your trying to get more storage cheaper and don't mind a higher rate of failure. If your data is at all valuable, and it almost always is, redundancy should be added as well.
And as for running Linux, if it can't run Linux I wouldn't want to own it.