Budget server config

[Kaassouffle]

Budget (including currency): €150~200, as cheap as reasonably possible

Country: Netherlands

Games, programs or workloads that it will be used for: NAS, Nextcloud, Home Assistant, MotionEye, Discord bot, maybe more in the future

Other details (existing parts lists, whether any peripherals are needed, what you're upgrading from, when you're going to buy, what resolution and refresh rate you want to play at, etc): 

I currently have a server with the ASRock Q1900M, a motherboard with integrated Intel Celeron J1900 from 2013, 4GB DDR3 RAM and I want to upgrade. I have a cheap 250GB SSD as boot drive and I threw in a 1TB NVME drive (Crucial P1) that I had left over from my main PC that I use for mass storage. I don't need more than 1TB at the moment.
I would also like the server to be energy-efficient.

 

I've found a couple CPUs second hand:

Ryzen 3 1200 for €65

Ryzen 5 2600 for €90

Ryzen 3 3200G for €94

Ryzen 3 2400G for €105

Ryzen 5 3600 for €125 (bit too expensive)

I think the 2600 would be the best value, but then I would need a GPU for a display output. I probably don't need it and do everything through SSH, but it would be nice to have in case SSH doesn't work or when I know how to do something in the GUI but not how to do it through CLI. In what other cases would I need a GPU?

 

If I need a GPU, I have found a refurbished Dell Radeon R5 240 1GB for €35, or secondhand Nvidia Quadro FX 580 for €14. I'm not sure what would be better, and would probably suck when they actually need to do something other than displaying the desktop. But I don't know when I would need a GPU for something other than a simple display output.

 

For the motherboard, I've found secondhand the MSI B450M-A PRO MAX for €35

I've also found the ASRock A320M-DGS with a broken PCIe slot, so I would only have a small PCI-e 2.0 x1 slot. I don't know the price of it yet. I'm not sure if it would work the CPUs that I listed.
For RAM, I've found new G.Skill F4-2400C17S-8GNT for €30 (8GB, 2400MTs)

 

I already have a PSU and case (an old one, if you know any cheap NAS cases, please let me know)

 

Would this be a balanced build? What CPU would be best and do I need a GPU? Thanks in advance.

[OddOod]

For that pricepoint I'd recommend just grabbing some 4th gen i7 SFF from a local office liquidator and add your storage to it. 

[Kaassouffle]

I've found a Dell OptiPlex 5040 SFF for €175 (i5 6500, 4GB DDR3L), which sounds like a good deal. It was €650 in 2017-2018.
I'm a bit concerned about power usage. I want it to be quite energy-efficient (which is why I currently have a mobile CPU) and I recently bought a 80-plus gold PSU (I bought it recently, I couldn't wait until this upgrade because the old one broke). The 80-plus gold PSU is probably way better than the intergrated one, but would the integrated one be reasonably efficient? Could I move the internals into my old case and use my own PSU? I've searched for some videos on youtube and it looks like it uses a standard CPU power cable but I'm not sure. Also I couldn't find any cable for powering the motherboard.

Another question, in general: how much power would this Dell and the Ryzen 2600 draw while idling? My current Celeron draws 0.5W while idling which is negligible on the electricity bill (without accounting for RAM, motherboard and storage energy usage, not sure how much that would be).

[OddOod]

To get the real hard answer to power draw you'd likely need to get it and hook it up to a meter, but most idle pretty low. You can dig through the mountains of documentation available for the model (like this https://i.dell.com/sites/doccontent/shared-content/solutions/en/Documents/opti_5040_d11s001a.pdf) 
As for powering it with your current PSU, no, not without some (inadvisable) surgery. Dell runs proprietary power connectors which people have figured out and you can rewire a connector, but I'd recommend not doing that. (see image below)

If you want absolutely lowest power sipping possible, an rPi 4b would probably be the best option as it sucks back 20W under full load, let alone the 4W it consumes at idle

 

[Kaassouffle]

I really want x86_64 so I won't buy a raspberry pi, I'll probably buy the Dell and sell my current PSU. Thank you for your help!