My current favorite keyboard is a Unicomp Mini M, and my collection includes a couple of IBM Model Ms, which I grew up typing on.
It's basically a slightly nicer mechanical tenkeyless than the original IBM SSK: lighter, slimmer, native USB, and more simultaneous keypresses, but still hefty and solid, with the same buckling spring action and wonderful curved backplate.
Unicomp's brand new Model M keyboards are also quite affordable by custom keyboard standards: $125 for a New Model M, $150 for a buckling spring Mac keyboard, and $150 for the excellent Mini M.
I have some of the more common, modular custom keyboards with the Cherry-compatible switches and all that. By and large, they're more expensive and more work to get into an agreeable actuation force range if you're used to dealing with real buckling spring switches. At the same time, the actuation force curve is never quite right because the 'tactile bump' of such a switch always involves a brief spike in actuation force instead of the regular increase and then collapsing action you get with a buckling spring.
If you've never used a buckling spring keyboard, you may find that you prefer some of the newer switches, especially the ones that don't imitate the 'clickiness' or heaviness of IBM/Unicomp buckling spring switches. But IMHO a brand new buckling spring keyboard is not just cheaper but plain better to type on them any of the more expensive, more 'modern' options.
I happily used my Unicomps for more than 15 years. They're still fine but I got a Model F reproduction a couple of years ago. It's much more expensive but the feel is even better!
