The usual SLA is not expressed numerically but as a feeling. And a normal server suffices to deliver that feeling. I'd bet 99% of apps are used by under a hundred people and if they have to take a day off due to a head crash, it's annoying but not catastrophic.

If you have two sets of hardware the server can run on (cold standby), and RAID, and are competent at physical IT work, you can have faulty hardware replaced in an hour. Drive fails - replace it. Anything else fails - swap the drives to the other machine, boot it up and then troubleshoot the original.

Most likely you don't even need that. If the app server runs on a standard platform like Windows you can shuffle it over to some spare tower PC. You hear horror stories of dusty towers that nobody knows what they're doing - the horror there isn't from running server software on a tower, but from unmaintained servers no matter the form factor