I love my dropper personally and would not want an MTB without it. However is it absolutely crucial and cannot live with out it? Not really it is just a really nice thing to have and easy to obtain in various quality levels but the important thing assuming you get a decent quality post is the lever, spend good money on the lever have had too many people come to me and say yeah my lever sucks or have had to try and fix bad levers and replace a bunch of them. My Paul lever was not cheap but it works amazingly and there are other great levers but a lot of stock levers lead a lot to be desired.

If the frame is decent it will have the ability to mount a dropper so buying a bike that comes with a dropper is not as needed but generally a good quality bike will come with one. Really it should have an air fork, thru-axles and ability to mount a dropper. If it comes with a decent one great but so long as it can mount one easily and internally then you are good.