I have an ISR4331 that I’m using to replace an average branch office environment. You need to watch out for a clock issue, but not all of them are affected by it and a reputable seller is a must. Most also include the licenses you need so no extra cost there unless you need more services. I don’t run a FW yet because my lab doesn’t have Internet access, but I’d go for something from Sophos.

Air gapped, no Internet access. I don’t use Internet services for any of my stuff though so I can get away without direct Internet access

My optiplex 9010 sff is what I use for experimenting with services and as a staging area for moving VMs to my main lab because it’s air gapped. At max load it runs at 140w but it has a GTX 1650 that I use for gaming as well.

Otherwise the rest of my lab is only turned on when I’m using it or forget to turn it off when I leave the house. When I get a laptop again I’ll leave it on more. None of it is more than $150 to replace though. It’s a Hyve Zeus, Cisco isr 4331, and a catalyst 3750x so nothing heavy, just a little loud.