If you want a new WiFi card, that system uses an mPCIe slot for that, so I'd recommend the Intel Wireless-AC 7260HMW.  Make sure you don't get the Wireless-N 7260HMW, since that only supports 802.11n; I have no idea why Intel used the same model code for two WiFI cards with very different capabilities.  Also make sure you get the HMW, not the NGW, the latter of which uses an M.2/NGFF connector.

As for your other plans, you have a 2.5" drive bay and a WWAN slot, the latter of which I think will allow an mSATA SSD, but I'm not sure about that.  But you definitely wouldn't be able to install 2x SSDs and a 5TB drive at the same time, unless maybe you were planning to buy an optical bay caddy that will allow you to install a hard drive into it.  If so, I would put a spinning hard drive in there rather than an SSD since I believe the optical drive's SATA port is slower than the others, i.e. 3 Gb/s rather than 6 Gb/s.  However, if you're going for 5TB, check the thickness of the drive.  Standard drives are 7mm or 9.5mm, but there are also 11mm thickness drives, and they may not fit.

As for whether you should "just buy a little Dell", smaller laptops definitely wouldn't allow 3 drives or even 5TB+ of total storage, so if that's a priority for you, then that wouldn't be a good option.