SQL Server 2012, 2016 or 2017 in any edition: Express, Standard or Enterprise. VSys is not sensitive to software updates/patches on the database, no applications other than the basic database need to be installed on the server, and VSys handles its own schema updates transparently.
VSys does not require its own hardware, or even its own database instance, and will work properly sharing an instance with other applications. Disk space utilized depends entirely on your data. It’s a rare database that exceeds 2GB in the first year or two, and it will grow primarily based on scanned/attached files rather than the number of volunteers.
VSys will happily run on a virtualized copy of SQL Server. It does not require its own application server either. It's a simple two-tiered client-server setup thick Windows client with SQL Server as the backend. VSys can also be used in a Citrix or Terminal Services environment.