Compiler farm for gentoo

I want to set up a server farm for the purpose of compiling large software projects such as gentoo. I'd like some suggestions for the server hardware. Can anyone with experience in compiler farms chip in? For example, should I bother with SSDs or are they too costly to justify any benefit?

How will you install gentoo on your server farm if you don't have a server farm to compile gentoo to install on your server farm?