Crucial is a good brand for RAM. Not need to buy the higher end Ballistic series, their "normal" RAM will be fine. I think newegg sells the 8GB sticks for around $40.
They also make very good SSDs, but Samsung's EVO 850 series is very good and very popular. People gravitate towards Samsung when it comes to SSDs. Installing a 2.5" SATA SSD means you need to replace the hard drive, but before that you need to clone it. Most SSDs comes with cloning software. But you will need to buy a USB to SATA cable to connect the SSD to the laptop to begin the cloning process. On the other hand a M.2 SSD will simply show up as another drive on the laptop.
M.2 SSDs basically comes with two different and incompatible interfaces PCIe and SATA; PCIe is faster. The Dell Inspiron 7559 uses the SATA interface so be sure to buy that one. 2.5" SSDs have a max capacity of 2TB (about $900 - $1,000), while M.2 SSDs maxes out at 512GB.
The actual M.2 SSD slot varies in size. 512GB SSDs are typically 22mm x 80mm in dimensions and is referred to as 2280. I think the Dell can handle 2280 M.2 SATA SSDs.