I’ve uploaded a great resource for TXV sizing from sporlan. You can also just call them up , ask for an applications engineer, and they’ll size one for you (although honestly i’ve had them undersize me twice now lol so maybe go one side larger).
For very low temp, you face big issues with hunting if you use a mechanical TXV. The off-the-shelf superheat controllers only function down to -60C so that’s a problem as well. You can either mess with getting a mechanical valve to work or write your own logic on something like a Siemens Logo (which is pretty easy honestly).
For ye DIY’ers looking at big compressors for cheap, go get 'em but beware of compression ratios that are too high. Multi-stage will almost certainly be required but it’s not as difficult as it is daunting.
As you mentioned, oil management is a concern for sure, especially with compressors in series. Read up on discharge side oil separators. If you’re building your own skid, you can afford $400 for a spiral separator and a solenoid for oil return.
Overall, great thread everyone!
10-10.pdf (3.2 MB)
Also, there’s a big write up on this thread that would be relevant to picking a random compressor and finding out how much cold you can get out of it: