Interesting - appear to be a number of people interested in this but no one with any experience. In it's own way that answers the question!
I think the question is too generic; I couldn't come up with a good answer for it.
Not knowing anything about your solution, it's design, what users typically do, any current performance stats from the actual deployment: not enough to go on.
Also: you haven't posted the specs of your current server, info about how it behaves, nor the specs of the HP box.
So all in all: kinda hard to give any recommendations.
I have 3 physical dev servers running in my office and 4 or 5 virtual ones. But I don't try to replicate the specs of some of my clients (some of them have 24+ core servers) because I could never generate the same load anyway.
One thing to note: a NAS is never a good idea for anything FileMaker: don't put your live files or live backups on it.
sorry for not being clear - the question was supposed to be generic i that I hoped it might promote a discussion and give others ideas.
For clarity though my server is a 2012 Mac Mini, 16Gb Ram, 2.3GHz i7, 4 cores. External RAID5 on thunderbolt. I am an independent developer creating solutions for clients from 2 users upwards in all sorts of different industries. My reasons for not wanting a high performance server are for the reason you state more clearly - I will never be able to recreate their load and so want to be developing on a lower performing hardware.
Microservers appear to have no option for a cpu with more than 2 cores, max i can find is a Xeon E3 @ 2.3GZ
Also i should have been more clear on the NAS, the suggestion was that it should be connected with iSCSI, completely agree that a NAS in normal configuration is not a good idea for anything FileMaker except as a destination for a copy of a backup.