What kind of Rear derailleur do i have and what RD should I buy as a replacement?
I think you will need a claw hanger adapter.  The old derailleur has one that appears to be non-removable, and the new one has a 10mm bolt that expects to find a matching hole in the frame, which yours won't have.
You probably want something that looks like this:

There is a sunrace branded one, but it looks the same in the critical dimensions as a generic pictured above.