The easiest way to ask a good question is to have it about a real problem. Lets pick this question apart.
From your question
"This hypothetical setup would be a vanilla Debian installation with nginx and such."
From the [FAQ]
you are asking an open-ended, hypothetical question: “What if ______ happened?”
Also from your question
How does this setup compare between a Raspberry Pi (maybe a couple of them for balancing), a desktop used for everyday tasks, and a commodity rack mounted server?
What sort of desktop? What sort of loads? Where do we reasonably expect a bottleneck ?
Would we be comparing a modern core i7, a still reasonably competent and realistically retired c2d?
What sort of rackmount server? A brand new 4u server running dual xeons and inconceivably huge amounts of ram? someone's old alpha we pulled out of a university dumpster>
In short, you've asked a hypothetical questions about hypothetical hardware setups, and expect it to be treated as a real question.
The 35 dollar + S&H question I'd ask is why not try it?. Its one hell of a geek toy and cheap enough to buy after mowing a lawn or two. Throw on a copy of raspian, try your setup, and see if its good enough for your needs.
