Medium sounds very small for a 6" tall person.
To be honest, the most significant "non-modern" aspect of the 2014 Heckler is the geometry, which you really can't change. And the fact that this bike is undersized makes it even more pronounced as MTBs have gotten a lot longer. For example, most people your height would be an a Large with a frame reach of at least 475mm. The Med 2014 Heckler has a reach of 402mm. That's ~3" shorter. Also, seat tube angles have gotten 3-4 degrees steeper.
If you do want to keep the bike (at least for a little while) definitely get a dropper. PNW is a good affordable choice. Unless this bike has internal dropper routing (I don't think it does), you will be looking at the Cascade model. Having the shorter seat tube generally gives you more clearance for a longer dropper, so you should be fine there. The PNW site has a good calculator for each dropper model to see how long you can go.
I'd hold off on the 1x stuff for now. I would hesitate to start spending much on a bike that is old geo and too small.
Also, a 10 year old fork and shock could be OK if it was well maintained (but most are not) or they could be trashed. I'd be getting those serviced before I started spending money on 1x.
You don't mention which build the Heckler has, so it is hard to say what would be worth upgrading.