Okay, so my file server will have two hard drives. Should I place the OS and all the files everyone accesses on the two paired together or should I use one as the OS drive and the other for the files? I can't afford three SSD drives so these are my options.
I would keep them separate, since you are limited to 2 drives. RAID 1 won't be of much help and isn't a reliable solution as far as backing up is concerned. If it's within your budget, an external drive would be a good way to go, that is, if you are concerned with backing up your files.
Echo that: RAID1 is a terrible substitute for a backup system, even though it is misused that way often.
RAID1 maintains an automatic copy of absolutely everything on the "second" unit in the 2-disk array. If it ever has a problem with one copy, it uses the other and then tries (or alerts you to do it yourself) to fix the bad copy by copying from the good one. As long as it works you have a sort of "instant backup". BUT so MANY of the ways that can foul up the data on a HDD (user error, viruses and other malware, power supply surges, etc.) are likely to damage BOTH copies of the data in the RAID1 array, and now you have NO backup! Add in things like fire and flood in your home, theft of your machine, etc. - ALL of these leave you with nothing if you rely solely on RAID1. So, BACKUP done properly and frequently, preferably with disconnection and off-site storage, is the way to handle that.
So, what is RAID1 good for? It is for applications where you cannot tolerate any downtime and the system must keep on working smoothly even when one HDD has problems, until some downtime and repairs can be planned. For example, I use a RAID1 array in a computer in our retail store's Point-of-Sale system (the modern cash register). Sales staff in the store have no skills or time to fix computer problems - it just HAS to work ALL the time! If it has trouble (and I inspect it for error messages periodically), I fix then in the evening when the store is closed. And in the meantime I have automated backups being made every night and taken off-site.
Because a RAID1 array uses TWO HDD units to save ONE unit's worth of data, it is twice as expensive per GB than a single HDD. OP does NOT need it.