-150 at 1kw heat load ? Well then you can foget about a 3 stager cascade ...you would need 4 stager cascade in last stage R50 refrigerant ...
i would go with:
1st. R452a /R404a/R507
2st. R170
3st. R14 ( or mix R1150/R50)
4st. R50
to hold souch of LOAD in last stage you need 3 phase compressors , first stage should be HUGE , power consumption will be so large that you need dedicated line for this cascade
i will looking to see the build progress!