Hi All,
I’m looking for a new homelab - 1U 14" depth - form factor is important to replace the old one.
It runs mostly light weight servers: nginx, MQTT Server, Home Assistant, etc
But I am looking to also move Plex and TrueNAS and possibly a windows vm to run BlueIris NVR.
Any recommendations are welcome, my budget is £1000 but also interested in understanding what would be the ideal setup using a 1u server.
14 inch depth with storage is going to be hard to find, mostly those are going to be more like 24 inch deep. HP DL360e with some drives might be a good option (Gen 9 or newer is better than Gen 8).
There are a few 1u short depth Supermicro servers that will hold 4 laptop size drives, but getting them with the drive cages is hard to find on the used market. For Supermicro I suggest X9 or newer series of main boards.
The 14" depth is going to be a challenge. If you are looking to build your own, the In Win IW-RF100/ IW-RA102/etc (1U | Rackmount Chassis - InWin) might be a starting point.I don’t think that the Wiwynn Lyra SV315 10 SFF will work (see Craft Computing I broke some stuff... Wiwynn Lyra SV315 Build - YouTube) with the WIWYNN LYRA-M SV315 10SFF BAREBONE CHASSIS LGA2011 DA0S6GMB8C1 INTEL S2600G motherboard (WIWYNN LYRA-M SV315 10SFF BAREBONE CHASSIS LGA2011 DA0S6GMB8C1 INTEL S2600G | eBay).
Since you seem to be in the UK, did you check out https://www.bargainhardware.co.uk ?
Thanks for that. Not seen bargainhardware before - awesome 
Your answers have helped a great deal in terms of narrowing in on what I need. I have a synology nas that serves as my media server. But running plex and an nvr is likely but not huge amounts of storage required. NVR only requires 48hrs of footage so 1TB is enough.
The key thing needed is the processing power and potential to add VMs. Further I am not planning on building myself and would prefer off the shelf options.
I was considering some link a mini pc: https://www.amazon.co.uk/gp/product/B09SZ53RH8
But prefer to have something rack mounted and looking for more capability.
The HYVE branded servers on the used market are typically Supermicro X9 series in a short depth and 1u size, but they probably only have a single system drive installed. Would be decent for a hypervisor in the lab coupled with an external NFS/SMB share for the vitrtual machines.
I actually have an HP EliteDesk G3 Mini PC that I bought to “experiment” with using Proxmox. It works okay but nothing like my HP DL360 G8 or DL380 G9 - but those are much larger (and longer) and power hungry. The model you spec’ed above has much more horsepower (and two NICs 
) which will probably work fine. Just get a 1U shelf and “Bob’s your uncle”. With electrical prices some days I would like to have something smaller than the Proliants. 
One other note on the mini (or 1 litre) PCs - you can fit 2 on a 1U shelf