Oh boy, a computer thread!
I would STRONGLY suggest investing in a GTX 1060 or higher as those graphics cards are the most well optimized for DX12 and UE4’s level of rendering. If you want fast build times CPUs are important, but if you want a smooth engine experience and fast rendering, you just need to have a stronger graphics card. I know some people are attached to Quadro cards, but try to avoid them if you can. The large framebuffer is totally unnecessary when rendering realtime graphics, and the GTX 1060 has 6 GB of video memory. For $250, that’s going to get you the most bang for your buck performance wise. If you’re working at 4K resolutions, it might be better to step up to a 1070.
UE4 uses a lot of memory: for a good workstation where you’ll be working with multiple platforms 16 GB is recommended, but you can run multiple applications fairly well on 8 GB. It’s going to be accessing a lot of stuff, so I would recommend DDR4 RAM at the fastest speed you can afford and a good SSD to improve boot-up, load, and streaming times. If you can afford a 2 TB SSD to dump everything on, go for it. Otherwise, it might just be a better idea to archive your old files on a separate physical drive at 2 TB and use the SSD for your most important projects, Windows, software, and the engine. I can have Maya, UE4, Photoshop, Z-Brush, Xnormal, many Google Chrome tabs, and more programs open without hitting 8 GB. But if I add light building or video recording to all that, I would definitely suffer from only having 8GB and the engine to work with. 1080p 60 fps uncompressed video comes to nearly 10 GBs a minute, so if you’re going to be doing a lot of that kind of rendering and you have the money for 32 GB of RAM, go for it. But by and large, PCs will handle memory compression/storage well enough on lower memory. I recorded many uncompressed 1080p 60 fps videos with just 12 GB of RAM and a 2 TB physical drive, it just takes some more waiting time for the memory to compress and yield a file. SSDs can go a long way for video output, too.
Some people like transferring projects to external HDDs to take projects with them. USB 3.1 standards are important to have available if you ever need to transfer several hundred MBs or GBs of data on external storage and you don’t want to wait forever. A motherboard capable of USB 3.1 and faster memory would definitely be a great investment.
Xeon processors are pretty popular for workstations because they have a lot of multithreading. I’m OK with i7s and i7 extremes, but if you have the money for Xeons, that can be much better for light building. Intel’s CPUs perform much better with game rendering and UE4 when combined with NVIDIA’s cards than AMD, but CPUs typically don’t do any rendering work outside of building lighting. Your second-to-second experience is not going to be based on the CPU, but if you’re going to be going into the engine files to crank up the light quality and build frequently, then you will need the best CPU you can find. Otherwise, light building at an overcranked quality can take several hours. An ideal workstation is Windows 10 with DX-12, NVIDIA 10-series GPU (at least GTX 1060), Intel Xeon CPU, 32 GB DDR4 RAM @ 3200 Mhz, 2 TB SSD, and at least a 1080p main viewing monitor. You can go lower on the memory and pair a smaller SSD to a physical drive, but I would not recommend anything lower for anything else.