That is, ah, pretty beaten up... especially for the money they are asking.
SBL didn't make a whole lot of those machines. Documentation is probably a little sketchy but it is likely academic anyhow. The machines were not self contained; they required a PC. That means a traditional IDE slot (I don't think modern motherboards even have those anymore!) and, worse yet, the software that runs on the PC. 95% of the time when something requires a PC, the computer is "lost". And if it isn't, the thing is so pathetically out of date that you will bang your head against a wall with how slow it is.... Or you don't have the password.... Or it is Windows 3.1 and the drivers died but nobody has supported that in 15 years. You get the idea. Oh, and SBL stopped making those years ago so I don't think there is any support for them at all. About 10 years ago, there was one guy who apparently was very familiar with the machines, but I think he (and maybe SBL itself) is long gone. I wouldn't count on any parts, but there are some great foundry topics here on HMEM!
The long and the short of it is that you should probably look at something like the 612 as a retrofit project right from the get-go. Manuals would be of interest only at the most cursory level (you might be able to take advantage of some amp wiring, the mechanical assembly information, etc). Anything related to CNC operation would be effectively useless.
I may have some manuals (or access to) for the 612. I'm not sure. I've always thought it was kind of a cute machine, although I think the Emco Maier EmcoTurn120 and/or Compact 6 (same machine, different names) is probably a better base machine... but has some retrofit complications that make it *much* more difficult).
Retrofit wise, there is Mach, LinuxCNC and BeagleBone (I know almost nothing about the last). There is a plethora of "all in one" stuff being sold out of China now which is interesting to me. I'm tempted to get one sometime because they are ridiculously cheap.... But, therein lies the problem. Most stuff I've looked at from China like this is plagued with problems and has absolutely zero support. The scant (and I do mean s-c-a-n-t) manuals are so full of unintelligible Chinglish that they are effectively useless. And even the UI suffers from the same problems. They crash, some buttons are in English, some in Chinese, those in English leave you wondering what the intent was, etc. I honestly don't know why the Chinese manufacturer's don't pay some native English speaker some money to review/edit/write some stuff that is useful. But don't get me started on that.... The point remains, that you can try one of those cheap all-in-one controls and you *may* have good luck... or you may not.
From my perspective, Mach appears to have fallen out of favor. I've read a lot of people griping about Mach3 crashes and poor support from the entity that took over ownership of the product. There seems to have been a push towards Mach4, but it has a completely different pricing structure and appears to have its own stability problems... It is all a little strange because it seems like Mach adoption has changed trajectory (but, this is just my view... not sure what others would say).
LinuxCNC is an excellent package but it isn't for the feint of heart. Converting a machine like an SBL612, however, shouldn't be horribly difficult.
OK, I've blathered and blithered on far more than may possibly have been asked for or desired. So, I guess it is time to stop. If you want to talk more about this stuff, however, just ask. I'm happy to discuss it. FWIW, I own a stock EmcoMaier ET120 lathe, and a Hurco KMB1 mill that is going through a fitfull conversion to LinuxCNC using servos (not steppers). The latter, when done, will support 5 axis but I'm aiming at just 3 for now.