For this achievement you either need to buy 2 of the 4 mods for Old Man Winter or Radar Range.
These are:
Winter Boost $125
Accelerator Mod $150
Winter Mod $225
Slow Cook Mod $200
For the 2 complete sets, they are $350 each; however, the achievement stipulates that ANY 2 upgrades are fine, so I strongly recommend you get Winter Boost & Accelerator Mod for $275 as this WILL count.
Currency is scarce during this DLC, so save your money from the start and don't spend it on ammo unless you are really struggling. If you're playing on easy, melee kills normally suffice as enemies can shoot the broad side of a barn.