Apologies in advance for rambling on so much, but this is a bit of a complex question as the information is scattered in little tid-bits across various sources and most of it is more implicit than explicit, so open to a little interpretation.
Hmmmmm... that brings up an interesting question that i'm not sure has ever been address (at least in "canon"....) Can a Force ghost appear to just anyone? Does that person have to be at least Force sensitive or a Jedi? Also, even if Rey becomes a Jedi, could she see the Force ghost of a Jedi she did not know previously among the living? Luke makes sense... Obviously.. Maaaybe Yoda too, if he is with Luke... But not Obi Wan... She has no point of reference for him... And more to the point, Neither does Luke.. It would be really weird to see Luke's ghost talking with Ewan's Obi Wan, even with old Ben makeup on... It would take me out of the movie...
Now... All that said... I would be open to seeing Anakin appear before Kylo... Since they could not have a proper physical confrontation, their battle could be one of chewing scenery!
It's a bit complicated, but I think there's a number of factors involved in this sort of thing, from the sensitivity and training of the person being "visited", the intent and abilities of the consciousness making contact from the netherworld and even the location of the encounter.
First off there seems to be varying stages of a "force ghost" manifesting, each more tangible than the last, and presumably the more tangible they are, the less force sensitivity a person would need to have to perceive them. Sometimes it's just a disembodied voice, sometimes a spectral projection and others a solid yet luminous manifestation.
For example, in the Yoda/Dagobah arc on TCW we saw Yoda communicate with Qui-Gon, who claimed that he could not appear physically and it took a lot of doing on Yoda's part to make contact. In that instance, the key component seemed to be proximity to a nexus of the force (Dagobah in that case.) This seems to track with Qui-Gon's appearance on Mortis (which was visible, presumably because Mortis is a force nexus turned up to 11) and how Yoda was able to contact and appear in various forms to Ezra, Kanan and Ahsoka in the old temple on Lothal. It's been stated in various places that Jedi, Sith and presumably other force centric religions tend to build their temples on such sites, so that also tracks.
Recently there was a short story where Qui-Gon appeared to Obi-Wan on Tatooine while waiting by the burnt out sandcrawler for Luke to come back. There are two interesting elements to this, firstly that this particular manifestation wasn't just an image, but a physical body (bones, blood and all) manifested out of thin air. Secondly it indicates that a consciousness can continue to learn while in the netherworld, since this is something Qui-Gon previously said he could not do.
That last part seems odd considering that Qui-Gon also stated that he exists in a place where tome has no meaning. However, on Mortis there's this whole bit about how The Son showed Anakin his future (yes, his *actual* future as Vader) to convince him to join him and The Father freaks out saying he's broken the rules of time (or some-such.) From that we might take it that higher beings aren't supposed to bypass causality when they interact with the material plane. Which in this instance may translate to Qui-Gon's learning process only seeming to take place over the course of time when viewed from a fourth dimensional perspective.
We see a similar progression with Obi-Wan's manifestations from the disembodied voice almost as soon as he "died", to the spectral image on Hoth that Han practically rode straight through, to the manifestation on Dagobah after Yoda's "death" that could move vines aside and sit down on a log. I don't think we've seen an example of it as of yet, but I suspect the latter can be seen by anyone because they're actually physically there in the material plane.
As for how people become force ghosts to begin with: a lot of this is covered in that aforementioned Dagobah arc on TCW but the short version is that it's not really a Jedi thing, but something a being known only as the
Force Priestess achieved a long ago. The Jedi may or may not have had this ability at some point in the distant past, but if so it's long been lost and forgotten. Qui-Gon began this training but did not complete it before his death, which is why it took him so long to return and why his body didn't disappear. He later contacted Yoda and sent him to the priestess who after determining his worthiness taught him to better commune with Qui-Gon and learn the secret himself.
After that, as we saw in RotJ Yoda passed some of the knowledge on to Obi-Wan so that he could also train under Qui-Gon. It's not clear exactly what happened with Anakin but I think it's been implied in interviews that he had "help" from the other side (presumably from Qui-Gon, Kenobi & Yoda.) It's a fair assumption that the same is true here for Luke.
Whether or not Rey could also potentially do this when she dies: it's an open question really. It's certainly possible given the precedent, but it depends where her story goes.
