RasPi. The 4 is.good but anyone will work and allow you to get a feel for ha. An old laptop or computer.is also an excellent host. NUC or NAS are also popular
Aeotech is good for usb zwave
Not sure about zigbee but yes, usb dongle needed.
Both zigbee and zwave integrate easily
If you have both zwave and zigbee then I would try to get a Nortek HUSBZB-1 USB dongle. It has radios for both.
Thanks - will look into those.
Out of interest - my parents have one of these (Dell Inspiron 3050) sitting in the cupboard - would this be sufficient to run HA well?
Yes. Install linux and run HA in docker container
I see Nortek is US frequency only so don’t think that will work over here in UK?
Now to work out how to get it from them during lockdown 
Any particular linux best? I’m familiar with Ubuntu - will that be fine?
Ubuntu is OK.
That depends on how you want to run HA. If you are only going to run Home Assistant Container or Home Assistant Core, Ubuntu is fine. Both of these methods will not give you Supervisor and the added benefits that come with it.
If you want the Supervisor and are planning to run on the Dell machine you mentioned, you have 2 options. Home Assistant Supervised or Home Assistant OS running in a VM using some like Proxmox.
See this post for an explanation of what each method gives you.
Clear as mud 
What is an add-on in HA terminology? Seems if i’m not supervisor then I wont be able to install add-on’s? Sounds fairly limiting, but i don’t actually know what an add-on is…
Ubuntu works
I prefer docker install
Add-ons are basically services that you can run like nginx, let’s encrypt, etc. If not using add-ons you just install them separately. For me the addons add a layer of confusion…for others it simplifies the process. On dedicated HA servers I run add-ons, if not dedicated to only HA I do not.
Supervisor gives some benefits, like easy upgrades, snapshots, add-ons that simplify life (e.g. google drive backup).
Just found an old Samsung NC10 in my wardrobe which I could start setting up immediately - can anyone confirm this this is sufficient to run HA successfully if I were to get a Zigbee/Zwave adaptor?
Yes
It should be good enough to start on
Thanks. I was reading over here that I might have issues running Supervisor due to the 32bit nature of it… ?
Just been perusing the forums and other than ‘easy backup’ and ‘add-ons’ i’m not finding much in the way of a definitive list of what I can / can’t do with or without supervisor
Backup / snapshot is clear to me (but equally seems I can one off backup anyway?)
Re. add-on’s this term might mean something to an existing user of Home Assistant, but it’s not entirely clear to me what is and what is not an ‘add-on’ as a new user
I want to run Zigbee and Zwave devices, likely via ConBee or similar, plus some wifi devices etc - are these ‘add-ons’? I come from Homey, over there these would need to be ‘apps’ - is that paralleled?
No. Zigbee and zwave would be considered integrations. I believe conner has an integration as well
Integrations are software based services that connect you with your devices through APIs
EXAMPLE: ip camera, Roku, Samsung TV, Hue
An Add-on is an external software service that runs in docker containers. These services can be managed and confirmed in HA supervisor.
EXAMPLE: ssh, nginx, let’s encrypt
The NC10 had a 32 bit Atom processor, iirc. So, all docker-based installs are out. In fact, I would suggest you get yourself a pi rather than spend time on such a dated hunk of hardware.
Yeah, leaning that way.
Actually, leaning towards the Dell Inspiron 3050 I mentioned above - will just need to wait a little longer for my parents to be out of lockdown!
Also, realised that I do want Supervisor as I was hoping to run my Plex Media Server on the same machine and seems there is an add-on for that which might simplify life
If i were to buy a Raspberry Pi for this - worth splashing for the 4GB?
They suggest 2gb here Install Home Assistant - Home Assistant (home-assistant.io)…