I've seen a lot of people running their routers as a VM on something like Proxmox and that gives you even more flexibility but it does require a beefier server - one that could potentially replace all the RasPis, potentially making the PXE boot redundant. :D
Home Assistant can take a minute or two to start up, keeping all four cores quite busy, so I wouldn't recommend trying this on an older model myself.
If you want to use the Pi's internal Bluetooth module, obviously you'll also need one that has one and supports Bluetooth LE. Again, I can only speak for the Model 4 here, which works great for that.
In practice, you probably want at least 3b+ for reasonable performance. If you're buying new anyway and can get them for MSRP I don't see why you wouldn't get a 4.
But, if you are looking for cheap and relatively low-power compute, I strongly recommend looking at used ultra small form factor PCs. You can get much more computational power and expansion, often for cheaper than a Pi. And eBay is riddled with these things, unlike recent Pi availability.
https://www.ebay.com/sch/171957/i.html?_from=R40&_nkw=%28len...
The pricing gets even better if you want to buy them in a lot.
And here's a containerized version that works with the Pi: https://github.com/1activegeek/docker-airconnect
- The excellent Home Assistant, for unifying across Homekit and Google Home and tracking historical temperatures and a couple of automations. The RPi has Bluetooth built in, so I can capture the data from a few Bluetooth thermometer/hygrometers running custom firmware (https://github.com/pvvx/ATC_MiThermometer) without a 802.15.4 bridge or similar.
- An AirPlay to Google Cast bridge, mainly for listening to Overcast or the occasional YouTube video on Google speakers (without subscribing to Youtube Premium/Music)
- A SMB server, for file storage and potential Time Machine backups (but I don't currently have enough storage, and locally attached SSDs are just hard to beat in terms of performance)
- A DLNA server, for watching photos and videos on my TV
- Tailscale, for the occasional use of my home connection as a VPN when traveling (really glad to be having symmetric fiber for this!)
- Caddy, as a frontend for everything web facing, to benefit from its excellent Let's Encrypt integration for automatic certificate requests and renewals
Most of this is running in Docker containers and configured via Ansible, so that if the microSD card burns out (or I botch an OS update), I can just flash a new one with an empty image and recover from there.