Nolan,
re: your question about ArcSDE cost....depending on the version of software you are on (and assuming it is on maintenance), ArcSDE is included with ArcGIS Server in most cases (but check with your local ESRI rep or distributor to make sure). you can read more about all the forms of geodata What is Geodata including the various flavors that are available. I am most familiar with MS SQL, so have no suggestion on PostSQL (but there are threads that talk about it)
Then it seems that maybe you need to look at creating a arcpy script that will automate the reading in of the spreadsheet, creating the events and a feature class (I recommend the file geodatabase), appending or replacing the database that feeds your service and refreshing it. You could set this up to do it at a specific time (weekly, daily, hourly). That is one option.
Another would be to create a front end that allows the users to modify the database, preferably a "version" of the database, and then reconcile and post at a specific time.
Sorry I don't have any code specific code to share that shows you how to do all this, but there are probably samples of similar workflows on Geonet and the web.
By the way, it may help to know what version of the software you are using since their are subtle differences between them, and of course what is available licensing wise. I would suggest 10.2.x or higher