Just buy and use the miniatures and other terrain. Find an easier rule set or just write your own basic rules.
Easy to say but difficult to do. But it's worth it. I ran a house-rules zombie game that was a huge hit. The focus was on the minis and terrain, and the rules ended up being 2 pages long. Yours might be a bit longer because you have to accommodate some "fallout universe" stuff in them like perks, when the mysterious stranger shows up, etc.
Most wargames have 4 sections, each of which will have to contain rules governing the various tasks and defining what it takes to achieve certain effects. These main sections are: INITIATIVE - who goes first MOVEMENT - how far, where, movement penalties etc FIRE - Shooting and various other ranged action things MELEE - Hand to hand combat and other up close and personal actions. Then it starts over, and round and round she goes.