Do you mean post-apocalyptic, or post-post-apocalyptic??
If I had ~100 dollars to spend, not including shipping, this is a sample list of post-apoc purchases I would make:
10174 Imperial AT-ST $49.98
8379 Pirate Survival $3.49
7625 River Chase $19.99
7991 Recycle Truck $14.99
8958 Granite Grinder $9.99
TOTAL: $98.44
Notice the synergy of the Recycle Truck with the Granite Grinder. The Imperial AT-ST can be used to build up the other vehicle sets (lots of very useful gray parts), or to build a fortification. You can build several vehicles here, though I think you'd be better off building 2 detailed and refined vehicles.
Here is another list of ~100 worth of sets that I might pick up as post-apoc fodder:
6241 Loot Island $19.99
8630 Mission 3: Gold Hunt $29.99
6743 Street Speeder $12.99
7621 Indiana Jones™ and the Lost Tomb $19.99
7901 Airplane Mechanic $3.49
7901 Airplane Mechanic $3.49
7668 Rebel Scout Speeder™ $9.99
There are some nice pirate and star wars elements here for you to build a band of thieving post-apoc brigands. There is synergy between the loot island and Indiana jones sets, allowing you to build a desert-themed hideout. You can even build aircraft here, or simply take the engines from the airplane mechanic and strap them onto your vehicles for high-speed ambush automobiles.
TOTAL: $99.93
Another list of ~100 dollars worth of post-apoc goodies:
8259 Mini Bulldozer $9.99
8259 Mini Bulldozer $9.99
8632 Mission 2: Swamp Raid $19.99
7622 Race for the Stolen Treasure $29.99
7631 Dump Truck $19.99
8014 Clone Walker™ Battle Pack $9.99
TOTAL: $99.94
A decent half-track building post-apoc group with nice military variety in figs and weapons. The two Mini Bulldozer's have some nice wide treads which can be used with the Race for the Stolen Treasure set to good effect.
One more list of ~100 worth of sets:
7685 Dozer $39.99
7628 Peril in Peru $49.99
5611 Public Works $2.44
5613 Firefighter $3.49
5612 Police Officer $3.49
TOTAL: $99.40
This has lots of potential, considering the fact that three of the sets are impulse, so you can customize the figs to suit your creations. The Peril in Peru has synergy with Dozer due to the yellow elements, but the important thing is the great variety of post-apoc friendly parts in that come in Peril in Peru. Dozer is an obvious choice due to the treads.
Here's my breakdown on the themes and sets at S@H:
Your best bet is to pick up one or two Indiana Jones sets.
River Chase is a good all-around pick, but Race for the Stolen Treasure is also a good base set for military vehicles. Peril in Peru is decent, as is Shanghai Chase and the Motorcycle Chase sets. After that, the sets get more expensive or less useful, but all Indy sets have SOMETHING for post-apoc, such as the Temple Escape, Jungle Cutter, Fight on the Flying Wing, or the Lost Tomb.
The next theme to consider is perhaps Agents.
Mission 2: Swamp Raid, and Mission 3: Gold Hunt are the best for post-apoc.
Mission 6: Mobile Command Center and Mission 5: Turbocar Chase are not strongly recommended, but would add to the post-apoc building experience.
Star Wars is full of several good things. One of these is essential: Grey brick. I strongly recommend the Imperial AT-ST for post-apoc part sourcing, and while it's not the cheapest set available, it is currently discounted! Do not miss the Hailfire Droid & Spider Droid set, which is almost sold out. This set has a great supply of small brown chain/tank tread and it also contains many VERY useful parts, including hose nozzle guns. The Rebel Scout Speeder is a good cheap set that could help out.
City Transportation has one useful little set called the Airplane Mechanic.
The City Construction theme has a few nice items such as the Dozer which comes with tank treads. I think the Recycle Truck shows promise and is an easy source of orange. Dump Truck is also noteworthy for its parts and is a nice value.
I might as well mention the City Police Officer set, since it's a decent source to get yourself a dog.
Also, the tools in the Firefighter set are decent. I'm listing these sets with the assumption that you have little to no Lego and want to jump right into post-apoc building.
In Creator, I would suggest the Stegosaurus which is good for terrain mostly, and maybe the Street Speeder.
In Pirates, I think Pirate Survival and Loot Island could help a post-apoc builder.
Power Miners isn't a very good option for post-apoc, but the Granite Grinder is a good choice for rubber tank treads, and the orange elements could complement the Recycle Truck from the City theme.
Unfortunately the best set for post-apoc parts in the Castle line is sold out. That would be the Troll battle wheel, which has oodles of useful 2x6 dark brown plates, and black hinge bricks. You're better off buying Star Wars for your gray parts, I believe.
Good luck with your post-apoc aspirations!
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