critical network hardware support basics

I’m trying to get a basic understanding specific to critical network hardware, such as host servers, firewalls, SAN etc – e.g. when you purchase such equipment, what kind of support/cover do you include in the purchase? (or should you). Or is it common in the industry to just purchase the hardware without any specific cover/support agreement?

For absolutely critical network hardware like your host servers, storage, firewalls etc – what kind of cover/support if budget permitted, would be considered ‘best practice’ and what kind of upfront/annual costs are included with such a deal above and beyond the cost of the hardware?

And in addition to the actual kit and support, is there anything else (cost wise) that needs to be considered in the upfront and ongoing costs associated with owning such equipment?