Those are actually (I think Fujitsu) Workstations. You should expect the same downtime you would expect from your own workstation if you were to use one at home. Maybe a bit less downtime since there would be less startups and the temperature differences would be smaller as well. Obviously, server grade hardware will perform and last longer under heavy loads.

I believe if you don't go with the Server auction thing, then you get quite new hardware. So I guess you would not really have to expect a lot of downtime. But as always, downtimes can happen. Always plan accordingly.