Shall we create a Database instance on an existing SQL Server or it requires Dedicated SQL Server? Yes
How to limit Agent installation only to client OS?
What do you mean? It all depends how you are going to deploy the Agent, via AD or Unmanaged Device discovery, in both you'll need a way to differentiate the OS type, since and inventory hasn't been run.
Limit Agent-less Scan only to Client Windows OS? Same as above
What is the Minimum Hardware requirement for installing only Inventory Component?
Check this: How to Run Inventory Scanner on Devices Without an Agent
Any other best practices guide available?
Installing an Agent and configure each additional component to the minimal would be easier, but you can do it as mentioned in the previous link.