S.K.
I am kind of biased, because I am not a big Brother fan. I seem to be the only one in that circle. I have helped a few of my friends with their Brothers, and I have always found networking it to be an issue. FYI: This is my experience. Everyone else on here has no problem. I have also read many posts on here where the Brother was shared from a computer using a USB connection, which I could never do. I think part of the allure of the Brother is also the downside (IMHO) is that it is a good value for the money.
Your 9840CDW is Ethernet compatible and, therefore, should be connected that way so that everyone can connect to it easily. You should be able to everything, e.g. print, copy, fax, scan, scan to fax, scan to PC, and both fax to PC and PC to fax. The only question is whether the Fax to PC delivers the fax directly as a PDF file and not a tiff. I know it can be converted in Paperport, but if that were the case, it would be an issue for me. Your model is a step up from most and may be able to do so.
I emailed Brother just now asking them to comment on their MFCs and on their 9840 in particular. I am supposed to get an answer within a day.
If I were you, I would plug it into an Ethernet jack and share it out from there. You could also call your representative and ask about the best way to Fax to PC and network.