Excellent, thank you.
For those reading this, GatewayMT is the Storj S3 gateway (took me a minute or two to recall that). In short, if you upload files using the S3 gateway then MD5 checksums will be used in the same way that they work at S3. I confirmed this by manually uploading files with specified part sizes and verified the checksums using the method described here. It’s reassuring that uploaded data is verified in transit between me and Storj.
Thank you for answering my other questions. Your answer, as well as some additional testing has me reassured that the standard methods of selecting storage nodes will be sufficient.