1. is this software combination is suitable for the above mentioned purpose (Windows Server 2012 R2 Standard Edition, Sql Server 2012 Sp1 Standard Edition and System Center Configuration Manager 2012 R2 Standard Edition)?
The combination can be created or sofwares to be installe once the OS has been deployed on the client. Use Task Sequence steps for the installation of applicaiton post OS deployment.
2. Should i install a Standalone Primary Site to support 300 client machines?
For this you need to understand the Hierarchy of the SCCM, however as far as your setup is concerned as you are proceeding with deployment at one site it should work with Standalone primary site.
NOTE- Bandwidth should be consiered before implementation.
3. should i install all the components (SQL Server and System Center and Distribution Point) on a single server? if yes what should be the specification of the servers? if no then how much servers i needed and how much memory, cpu and disk space is required to accomplish above mentioned tasks?
Deploy the Roles that are necessary for OS deployment only(Mgmt. Point, PXE Point, WDS, WSUS, etc) refer to Microsoft recommendations.