I'm planning to buy a used K2 with serial numbers between 4900 and 5000.
I would like to ask what are the upgrades I need to make it equal to the current K2.
The last major mod to the K2 was for S/N 4060 and above using Revision B PCB, so you should be OK with S/Ns above 4900. There have been a few other worthwhile mods such as setting AGC level and the Magnetic Level Shielding Kit but these are not absolutely necessary for most applications. LA3ZA has a very nice summary of most K2 mods with links here: