Don’t know what temps you need to hit, but for a hellacious amount of cooling power at -50C I’d be looking at Fluid Chillers AIR ULT series, thats what we use to cool down our bulk ethanol.
In case anyone reading this hasn’t already seen it I’m the guy behind the Who sells huge cryo chillers that aren't huge disappointments? thread…I’m intimately familiar with what the Unistat line costs and how well they perform.
If you aren’t dissuaded by that thread or what @Tech1145 wrote and still want a Unistat, I have one Unistat 815 left that has a new ControlOne a couple months ago. Asking $29k OBO, currently located in northern Colorado but can crate/palletize and ship LTL for reasonable.