The first thing that you should do is see if your server supports Nvme drives and which bays do. If I’m not mistaken not all R640 have bays that support Nvme.
If it supports Nvme drives then it has a backplane with U.2 connectors. Both U.2 and U.3 drives are compatible. These type of drives are enterprise only, and unless something has changed recently, consumer grade 2.5" Nvme SSDs don’t exist.
New enterprise SSDs are very expensive (used are hard to find) and they’re only worth it if you’re going to write a lot to the disk.
Apparently there are m.2 to u.2 enclosures, which would allow you to use cheaper consumer drives. I’ve never used them, so I don’t know how good they’re. They may be total crap, so do your research before you buy such an enclosure.
The last option is to buy consumer grade Sara SSDs. This is the cheapest by far right now and probably the best idea unless you know you need faster drives. Sequential speeds of almost 600MB/s. Compatible with Sata/SAS ports, but not with u.2 ports.
For drives up to 4TB I’d recommend TLC and if you want 8TB drives I’m pretty sure there’s only QLC.
There may be some sort of marking that indicates if the bay is only accepts Nvme drives on the front of the drive tray. Line this.
Another option would be to open the server and find the part number for the backplane on Google or Dell’s page.
U.2 connectors and sata connectors are pretty similar, so It will be hard to tell only by watching the connector.
The link you’ve provided is the type of enclosure I’ve mentioned, that goes from m.2 to u.2 . I’ve never used one of those before, so I don’t know how well they work, but there could be compatibility issues with some operating systems, especially if you plan on setting up RAID.
If the backplane ends up having SAS connectors, you could try and get used enterprise SAS SSDs. Sometimes they can be had for about the same price as consumer SATA SSDs. And the max sequential speed is 1.2 GB/s.