So if you destroy your vehicle it is done for the rest of the mission unless you restart the mission?
If your just crawling around in 1-Low you should be fine, but if your careless like that Tomcat Youtuber and running over rocks in auto, or if you somehow run off a cliff, then no your vehicle isn't done.
There is no garages in Old Timers, but your vehicles come per-equip with repair points, you start with 2 vehicles, you can unlock a Napco on the map (there is always one near the fuel station) that you can use for recovery, or you can unlock the DW950 which can haul around one of the many semi-utility trailers that is on the map.
When you start the map just pick 2 of your favorite Scout Vehicles, the Hummer and F-150 are good starters. Napco is good to but like I said there is always one on the map you can unlock, so you don't need to choose one at the start. There is no point picking a big-rig as you can't use those to complete this map, the wood carts can only be hauled by Scout Vehicles.