Actually, hitting the 5 9's for "reliability" and "downtime" with any decent piece of kit is pretty easy, and the vast majority of vendors when it comes to hardware failures these days far exceed that.
But, and it's a big but, you then get into five very common situations
1. You were one of the first to use those shiny new software features that had a bug that corrupted everything
2. You are dumb enough to take that system running fine for a few years, turn it off, and move it across floor to shove into different corner
3. The third party company you subbed the upgrade to didn't do something right during the hardware upgrade
4. You didn't replace that hardware item a few weeks ago when it first failed
5. Sooner or later, you get to a situation, where due to economy of scale, the guy on the phone from support has to tell the guy on the other end to replace something important and <human> happens
So you really need three things:
1. The actual hardware reliability
2. Average amount of time after major software release final HOLY CRAP bugs get worked out
3. An idea of how much your refusal to allow remote support is going to hurt you when you release the customer support engineer licks rocks.