Weapons and defenses
The developer describes a loadout in four words: “Add crazy weapons & defenses!” The and matters, and so does a slot rule that decides how much you can carry.

A roll pedestal and its label: Green Smasher, 1 in 14, Epic, $400.
Frame at 2:30 of I Built the STRONGEST CAR to SURVIVE.., recorded by Default — a player’s video, not an official image
The labels, as the pedestals print them
Metal Spike
1 in 9
Rare
$300
Not seen fitted yet
Green Smasher
1 in 14
Epic
$400
Uses no support slot
Double Barrel
1 in 18
Legendary
$2.2k
Not seen fitted yet
Rocket Turbo
1 in 25
Mythical
$2k
Not seen fitted yet
Crossbow
Label not filmed
Uses a support slot
Crossbow has only turned up by name in a build inventory, never on a roll pedestal. The odds and the word come as a pair on every label here; how the two notations line up.
Not everything costs you a slot
Build mode shows a support counter, and the skill tree has nodes named More Support. Watch the counter while two weapons go on, four seconds apart:
Green Smasher fitted0/2

Green Smasher already fitted to the front of the car and gone from the inventory. The counter still reads 0/2. Rocket Turbo and Crossbow are waiting in the inventory.
Frame at 2:44 of I Built the STRONGEST CAR to SURVIVE.., recorded by Default — a player’s video, not an official image
Crossbow fitted1/2

Four seconds later the Crossbow is on the car and gone from the inventory, and the counter reads 1/2.
Frame at 2:48 of I Built the STRONGEST CAR to SURVIVE.., recorded by Default — a player’s video, not an official image
The smasher rides free and the crossbow takes a slot, so the cost depends on the part, not on whether it counts as a weapon. A turbo or a spike has not been seen going on yet, which is why the labels above leave them open. Your own counter settles it for any part in about a second.
The half not on film yet
Defenses. The developer names them as their own category and no recording we found shows a defensive part being fitted or named. Every list read for this site, including this one, is really a weapons list.
No damage numbers either
The prices and odds above are what the screens show, and nothing shows how hard anything hits. That matters less than it sounds, because what the recordings show ending a run is an empty tank or a wrecked car, and distance is one of the two cash sources.