Not sure on an 873.....many variables. Age, maintenance history, stored inside/outside.....etc. Not sure I would trust the hours on any used machine as the meter can be replaced. Sorry if this isn't much help but giving it a thorough inspection and doing your homework in advance goes a long way. Run it through it's limits for sure (for example, is there excessive creep?.........can you tilt the bucket down to the ground to lift the front wheels?.......if it doesn't do that, or struggles, then I'd be wary)
For context, I paid 12K 2 years ago for a 1993 853 (in Ontario). Running condition with minor leaks (unknown hours). Needed a bit of maintenance (normal wear and tear) but if you do the work yourself and/or figure it out as you go, it can make it worth it. I've put 70 hard hours on this thing and it's still going strong. Best of luck with your search!