I think you could give the offer a shot, but it looks like the market is a good bit higher - the bigger the discount the more you're probably going to have to work to get it operational. It looks like you've already got a headstart on that, so if none of this is scaring you off, I say do it! My two machines are Whitestar and Spike2 systems, and are from Stern, which is still operating. My concern going for Party Monsters would be that System 11(B?!) boards and replacement parts may be in short supply, and I can't comment on the quality of documentation for it all.

Looking at pinside though, between the owner thread and system 11 thread, there are probably more than enough helpful folks who will have good advice for you if you take on the project. https://pinside.com/pinball/forum/topic/elvira-and-the-party... https://pinside.com/pinball/forum/topic/system-11-club

Watch out though - I see posts about mods in the game thread. My newer machine came with 6-7 already installed by the first owner, which got me hooked. At this point I've added another 25 or so... I think I spend as much time working on these machines as I do playing them (but my goal is to make my own machine, so every piece of work comes with learnings and better understanding of what's in store for me.)

Post back here later and let me know what you end up doing!