Base classes can level to 70, but you will not have any skills past level 30.

Jobs come after classes and if you level for example, Dragoon to 46, the Lancer will be 46 also. Jobs go up to lvl 70.

Machinist, DRK, and Astro do not require any pre-existing classes and do not have any bas classee. You just need one class at 50 and to reach Foundation in the MSQ. When you get those classes, they will start at level 30.

At any time you can switch jobs and even switch back to the base classes. You can continue the MSQ on any class high enough, or level Machinist to the required level if you want to use that for the rest of the MSQ.