ac 1450 confusion

I recently bought a Netgear AC1450 and want to upgrade it to have vpn support. I am still a little confused after reading for hours. I downloaded dd-wrt.K3_AC1450.chk but from what I understand that's just the initial flash I need that's specific to my router. What is the other flash that I need. If you can be as specific as possible. I have used dd-wrt for years so I am familar with the procedure but just unsure of the flash I need and don't want to brick my router. Thanks in advance.