You need to be careful of used DH bikes, a lot of them are for sale because they’re completely shagged out.
I’ve been mulling over the same question, in the end I’ve decided against it because I don’t think I do enough DH to either justify it, or to really learn the bike… Say I can get in one uplift, race or one push-up day a month at best, that’s just not much seat time and I’d end up wasting loads of it mucking around with setup or finding faults or just plain getting used to the thing.
Whereas my Hemlock’s perfectly capable of DH stuff, we’ve gone racing (badly) and we’re off to the fort william endurance dh in july, but also it’ll pedal up and can be used for all sorts of riding so there’s never any need to familiarise myself or adapt to it when I do go and do downhill stuff.
But, ymmv.