BP rifles as a rule are not that expensive new, and that's where I think I would focus if I were to get another BP rifle.
I own 2 BP rifles. The first one was a freebie, a rusted and crusted bag of neglect, and in pieces. If not cleaned properly, it's unbelievable how bad these can get. I did hunt with it, and harvested a nice spike buck. I later bought another used one, and it was in nice shape, no surprises. This one was $75, and I only bought it because it was the same as my rusty one. It shoots OK, and better than my old one.
I'd probably not pay more than 1/3 of MSRP for any used BP rifle. Less if I could see any rust. Do your research, some models are not worth owning, free would be overpaying.
Avoid the ones with weird bore/groove dimensions, like Gonic or White. The likelihood of coming across one of those is slim, but they are out there.
Stick with 50 cal.
Avoid any model not in current production.
ht tps://thegearhunt.com/best-black-powder-rifles-reviewed/
And the #1 choice is $500 at Cabelas.