I saw one of these at a local camera show a while back. I forget what they were asking for it, since I did not have an interest at the time.
I'm wondering if a DSLR (or in my case, a Fuji mirrorless) could be substituted for the included SLR, which I assume is some kind of a Pentax body.
I would suspect that it could. I saw one for sale on an auction site recently, did not bid, but it was similar to the one you are showing above - had a Nikon body though, which is why I presume the bidding got so high.
You can also buy something modern and made for the purpose, but it's not cheap.
http://www.filmtoaster.photography/
I presume the Film Toaster works very well, and if I were better heeled, I might consider one myself. Alas, budget rears its ugly head.
The requirements to get into this type of image copying are fairly simple, it seems.
You need a camera - the higher quality, the better, of course.
You need something to copy - negative or positive film, one presumes.
You need a way to focus on the film - this will work best, one presumes, with a high-quality lens, probably macro, and perhaps a so-called process lens or other type of flat-field lens designed for exactly this kind of thing - copying flat objects.
You need a way to light the image from behind. You'll want to make the light bright, diffuse, and cool enough to not cause physical damage due to heat to the film itself.
None of this is new - in the olden days, people used 'slide copiers' attached to their film cameras to do this kind of copying, but they were copying film to film - from slide film primarily.
This would still work with some caveats. First, many DSLR cameras are not full-frame, so an attachment designed for a 35mm slide to a 35mm film is 1:1 and a APS-C or M4/3 sensor isn't that. So the lens used would have to change, or the distance between the film being copied and the camera.
At the simplest, I have seen folks simply use a tripod with a DSLR attached, a macro lens, and a piece of film on top of a phone or tablet with the screen turned white, like browsing to a blank web page kind of thing. Some have said that worked pretty well for them, others have said it's not bright enough for dense negatives or it lets the 'dots' of the device used for lighting shine through.
Some have used a shoebox with a flash inside kind of thing. Sync the flash to the camera and you should get lots of diffuse light, one suspects.
I think it's a matter of experimentation what works best for you.
There are a ton of youtube videos on various ways to do this, and lots of web pages. Seems everyone has a different take on it.
Bottom line, though, the basics are still the basics. You're going to shine a light through a piece of film, and photograph that. Like an old fashioned slide projector in reverse - the projected image gets photographed by a DSLR instead of being projected onto a screen.