Before buying anything, check if the floppy heads are dirty by cleaning them. You will need to take the system apart and use a Q-Tip WITHOUT bending the drive heads these days, as there are no new head cleaners made that are designed to clean these drives made anymore.
If you keep having problems, you may need a new set of disks to get the system to boot with them. This can be difficult to make unless you have another M68k Mac (or a PPC system that can write these legacy disks) and a way to transfer the disk images to it so you can make the new disks. You MUST have a Mac with a 720k or 1.44MB w/ 720k compatibility floppy drive to write these disks. If you don’t have access to another Mac from this era, I’d just buy another set online to make things easy.
As to the drive these old SCSI drives have a rubber bumper that has a tendency to melt as the drive ages and this eventually gets onto the headstack and platters. The problem is once this happens, the drive is ruined and has to be replaced; it can be replaced BEFORE it melts but there’s no guarantee the damage was partially done when you change it, especially on a unknown used drive. You will need to patch a copy of HD SC Setup as the official version from this time period is known to only work with Apple firmware flagged drives; no other drives register to the unmodified version. You can still buy the Apple drives, but they are becoming rare and expensive and will eventually fail the same way. You’re honestly better off doing a SCSI->CF or SD conversion with an adapter and pairing that with a modified copy of HD SC Setup.