So I have a Lenovo X3550 M5 with 4 drive bays. It's more than powerful enough for what I want to do. It also only has 4 drive bays. I could add 2 in the rear, but the media cage is super pricey IMO. I guess the power draw would be pretty high as well? And noise will be a factor. I can put it in my basement with no problems.
I also have a Lenovo Thinkcentre M72E SFF that I'm considering using everything but the case and get a bigger case....one that can hold 6+ drives. Doesn't have to be 2.5" or 3.5" size. I will probably use a different PSU if I use the hardware. I could also utilize a bigger GPU as well. I have a 960 (IIRC) 2GB that is passively cooled in the system.

So if I use the M72E, I will need a new case that can hold more drives. I was thinking of using SAS drives just for the hell of it lol. Maybe a raid card too.

Comments? Recommendations on a case? Or maybe a different idea?

You did not post the purpose of the computer except maybe for storage and I don't know if the M72E qualifies as a HTPC so not enough information to make a recommendation.

SFFs like Dell and maybe Lenovo tend to have specialty cooling for their CPU and may not conform to ATX standards so putting the motherboard in another case my not be as easy as you think. You do need a bigger PSU to accommodate the number of hard drives.

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What capacity do you need? Your options will vary depending on how many gigabytes or terabytes you plan to have available on your media server.

Your proposed system will make a lot of noise and use a lot of power. There are now increasingly decent storage options for the Raspberry Pi 5. Although the Raspberry Pi 5 uses more power than the earlier models, it is still light compared to what you are proposing. Two possibilities which might be of interest might be Radxa's Penta SATA HAT or Geekworm's X1011 PCIe Four M.2 NVMe SSD Board. You'd be able to put OpenZFS on those.

Also, will you use tape or something else for backup?

Does power used support the use of the server?