Get the Forster CoAx-XL, which will handle the .375 CT. Hodgdon Retumbo is a goto powder for both, as is the Federal 215M primer. Brass is Lapua or Petersen for the 338, Petersen for the CT.
After that, it's a deep rabbit hole in what equipment is needed vs wanted. I use custom dies from Warner Tool for sizing, and Wilson inline seaters. If you go inline seaters, you'll need an arbor press, I use the K&M with force pack. I anneal in an AMP annealer. You'll need a brass cleaning system, I use both wet SS pins and dry corncob systems. A case trimmer system will eventually be required. A priming tool is also required, the CoAx does not have built in priming, and you need some precision there anyway.
I'm a big fan of the Autotrickler with the Ingenuity upgrade, but there are other electronic powder dispenser systems that will drop to the kernel, which is a requirement.
Get a Garmin Xero chronograph.
That will get you started

