If the hardware is a bit older and it is primarily used as some sort of server, I'd use Debian. Debain has the advantage that it's quite solid and easy to maintain (especially if you use the stable version). The drawback of Debian is, that you won't get much of the new shiny and flashy stuff ubuntu usually has and the multimedia stuff can require some work. But if the primary use is server and not desktop, that doesn't matter much.
The install shouldn't be too hard, although properly configuring the server functions might be harder, especially as Debian isn't great on documentation. A trick is to look at documentation of other distros (e.g. Gentoo) to see what configurations to make.
I only would use Gentoo if you want to learn a lot about Linux and have free time to tinker with the system. The documentation for Gentoo is great, but it isn't exactly easy to maintain. On upgrades stuff will break sooner or later, and although it's usually a quick fix, you have to invest the time to find and apply it.
And sadly it feels like Gentoo has been going downhill in the recent years.