Is it worth getting the MOC? Is it just like a bunker, but you can start supply runs on there and store cars?
It all depends on how you option it out.
You can make it a mobile temporary apartment if you want to hold up for a while without being attacked with room for a personal car or a command centre with external guns (rear mounted ones require research.) and a place to upgrade your weapons and Weaponized cars(You can only upgrade the cars in the MOC.)
You can start those special vehicle missions from it regardless of it being living quarters or command centre.
The MOC can also be upgraded to further improve its performance and durablity( much tougher than a Tank I like to believe fully upgraded.) and along with visuals like paints, The trailer too can be customised visually.
As for which truck to pick, testing by some youtubers found in terms of acceleration that they are practically identical, I personally went with the Hauler for two reasons, It's shorter in length so it'll be more agile should I end up in a tight spot and also it reminds of Goliath from Knight Rider.
All in all, If you have no plans of buying the new cars, don't buy the vehicle/gun workshop, if you do, get it.
As for if it's worth it all things considered?
I'd say yes it is, Because its still a good vehicle that's practical, you can detach the trailer for more speed and then use it in the Haulage Vip work, Fully armoured its tougher to kill than almost everything so it'll shrug off quite a few explosives and it'll be faster than any other truck in the game.
So if you're willing to put the money down on it, I'd strongly recommend getting an MOC. 👍