I need a mixed purpose computer, looking for suggestions

[DiTBho]

The HP Z-series are good, but power hogs.
Not sure whether that's important to you or not.

umm, 1Kwatt is the max I can supply.

I've already wondered why HP tends to make things so power hungry when I picked up my HP-PARISC workstation(1) and found that with GNU/Linux it draws up to ~ 150 Watts(2) even though there are nothing but common demons waiting for a connect, i.e. { sshd , telenetd, tftpd, nfsd, ... } but nothing that is actively but silently compiling inside a tmux/screen.

So, when Top says that both the CPU and IO loads are low, indeed you immediately get when Gcc, Ninja and distcc  are running because as soon as you try to invoke "emerge" to compile something serious (i.e. "emerge cmake", which eats up to 1Gbyte of ram, and takes 2 hours to compile), the wattmeter says that the power consumption has risen to 500 Watts, value in the measurement window of the wattmeter, which then goes back down to ~150Watt as soon as it's finished 

HP ...

(2) with
- 1 CPU, PA-RISC-v2@~550Mhz
- 8Gbyte ram (HP custom, I think they are ~DDR1)
- two SCSI HDDs@12K-rpm (Raptors)
- two PCI-fiberoptic cards
- one PCI-Wifi card
- one PCI-Matrox MillennumI, MDA
- one PCI-Adaptec-SCSI-lvd card

[Microdoser]

Question: why a server? What can a server do that a "regular" desktop PC can't?

Nothing. They are both just computers at the end of the day. A better question would be, in which way is a server more capable than a desktop PC?

The answer to that question is that a server generally can be fitted with more memory, that memory can be ECC memory, they have more PCIE lanes, the ability to fit more drives, better networking, and more cores/threads than your average desktop PC. So if your workload depends on multiple users accessing large amounts of data at the same time over a network, then a server will in most cases be far more capable of doing that. If you are a single user but want to have multiple VMs running many networking services, then a server will generally be more capable of doing that than a desktop PC. A single user that wants to have multiple cheap machines, all having access to the same data, and farming out the heavy lifting to a centralised machine running various VMs will find that having a server will mean a better and more efficient workflow.

Again, a desktop PC can do all of those things, but a server can do them *better*

[DiTBho]

I just decided that the killer app at the top of the list will be
QuickEmu! (click here)
kind of "profiler" for Qemu 

[Ice-Tea]

From what I've seen, servers or even workstations are stupid expensive compared to "regular" desktop PCs. If it's about space for expansion: get a mid tower or even a full tower.

Unless you need a server-only feature (like redundant PSU): get a desktop.

Putting a system together these days really isn't all that hard. Pick your parts, you can post them here for verification. I'm sure people (myself included) will be more than happy to double check. To check prices, I often use mindfactory.de. I'm sure there's a UK equivalent as well. For fun and giggles (non-researched, just as an FYI), I put together a system:

Inter-Tech B-42 Midi Tower ohne Netzteil schwarz: 29€
Generic mid-size housing. No rainbow barf
32GB Patriot VIPER VENOM DDR5-6000 DIMM CL36 Dual Kit: 108€
1TB Samsung PM9B1 M.2 2280 PCIe 4.0 x4 3D-NAND TLC : 90€
AMD Ryzen 5 7600 6x 3.80GHz So.AM5 BOX: 190€
Mid range recent CPU. Fairly future proof and upgradeable platform
ASRock Phantom Gaming Riptide AMD B650 So.AM5 DDR5 mATX Retail 170€
Full size ATX, more PCIe slots
2 x Phanteks SK PWM Lüfter - 120mm, schwarz 16€
650 Watt Seasonic G12 GM Teilmodular 80+ Gold 70€
No PSU redundancy, but a quality PSU goes a long way. Power budget for more SSDs, a GPU, bigger COU...

Total cost <700€. No OS. No GPU. There's GPU inside the CPU that is enough for "normal" productivity stuff including video conferencing and the like. I think the mobo will allow you to do dual display. If you need more: spend anywhere between 200 and 2000€