You could consider getting a LGA 2066 Core i7-7800X which costs almost the same as the i5-8600K. But with that you would have Quad-Channel DDR4 support on a X299 motherboard which obviously would be more expensive indeed but it is well worth the money. Gigabyte X299 UD4 Pro or ASUS TUF X299 Mark 2 maybe. 32GB DDR4 RAM would be better for UE4 development. And you could get a Nvidia GTX 1060 GPU which is not very expensive and is good enough to work with UE4 (obviously a GTX 1070 or a RTX 2070 would be better but way more expensive too)