I've got a GPS-disciplined Oven Controlled Crystal Oscillator (OCXO) hooked up to a raspberry Pi running Chrony on my home network. It provides a pretty good ti

I've got a GPS-disciplined Oven Controlled Crystal Oscillator (OCXO) hooked up to a raspberry Pi running Chrony on my home network. It provides a pretty good time source (and more importantly for me a pretty good 10MHz reference). There are rack-mount 1U NTP servers built for the purpose for about what you estimate. Most atomic clocks will have 20+ year lifetimes. OCXOs age out a good bit faster, but they're rather cheap (<$200 for ones good enough to provide holdover for a GPS module). TCXOs don't age as quickly (they're not literally in an oven), I'm not sure how long they can be expected to last.