In my experience a SSD makes a huge difference in building and working with UE4 in general. Both compilation and intellisense refresh involve reading/writing to quite large PCH files (1GB+). On a standard HDD, I/O becomes the bottleneck during most of the building process, now matter how fast your CPU is or how many cores it has. Having a lot of memory (32GB+) quickly demonstrates this: the first time you compile after booting the PC is very slow, while if you make a change and build again it goes much faster because Windows cached the PCH files in unused RAM.
So, the upgrade that would make the most difference is definitely an SSD, specially if it’s large enough you can install both UE4 and Visual Studio on it, as well as placing your project there.