SQL Server is a good database like MySQL, MariaDB, PostgreSQL or Oracle. The point really is if you have a control of the database or not. Usually for less 200-500K files with medium hardware it is enought to get OpenKM running with good performance without database tunning. But all it depends on your hardware, how users working with the application etc... there's not a magic receipt for suggesting a hardware configuration. For example with linux, always you will get better performance.
If you explain your case might be we can guide you. We need an explanation in terms of:
1- initial number of files
2- how many files will be created per year
3- OS
4- virtualized or dedicated server?
5- do you have some huge daily import process? how many users do you expect and what type do you have ( % of readers and % of writers )