The only time the fan runs on my 2018 15" MBP is on import where I'm importing to one external drive, copying to a second drive as a backup, creating 1:1 renders and also creating Smart Previews.....and it's not as loud as my 2013 15" MBP. Cool down is pretty fast.
According to the recommendations from Adobe, I'd get 16 GB RAM (I never ran Lightroom Classic with less, actually, even my mid 2012 15" MBPr has 16 GB, Adobe recommends 12 GB). Also, 256 GB SSD seems a bit small. Curiously, right now I'm pondering more or less the same thing as you, but I'd be getting 16 GB RAM and probably the 1 TB SDD. My biggest concern is the lack of a dedicated GPU.
I run LR6 on 8GB and have had no issues on my 2015. But you don't mention how large your RAW files are. If they're 80MB that's a different story to 5MB.
The processor gets hit hard when processing HDR or similar, but for simple edits in LR the processor isn't working hard at all and I never hear the fan unless I'm grinding through several HDR shots in succession.