Dell servers are as accommodating to 3rd party hardware as any, and there is a healthy after-market for upgrades and pulled parts. Just search eBay for “R7615 384GB” or similar.
You can plug-in most any off-the-shelf SATA or SAS drives, but that’s assuming you get a proper backplane with your server that supports the number and type of drives you want to use. If you want higher performance, an eBay search for “R7615 nvme” will give you some options.
As it happens there was a write-up on an unsupported method of using NVME in much older Dell servers, not long ago, which might be a good read for you about server mods and options, even if you don’t go that way:
There are limitations to servers in general… You aren’t going to find SATA power connectors dangling around in the case available for you to power accessories.
As for OSes, you’ll find a good array of options, and better Linux support than desktops or workstations:
Supported operating systems
The PowerEdge R7615 system supports the following operating systems:
● Canonical Ubuntu Server LTS
● Microsoft Windows Server with Hyper-V
● Red Hat Enterprise Linux
● SUSE Linux Enterprise Server
● VMware ESXi
For more information, go to www.dell.com/ossupport.