If you want to save money on things that aren't technically needed, you can also drop the CPU cooler (use stock cooler, it's free and it's good enough, let's be honest) and the case (air flow is nearly a myth, lol).
Getting a slightly higher quality power supply than you need opens up upgrade paths and ensures that it's not going to explode on you. If you want us to look into whether or not your current 500W PSU is worth replacing, please post its exact model. You might be perfectly correct, and it might actually be a gem that needs no further mention.
SSDs are nice. If you use programs that have long load times, chances are very good that an SSD will cut those load times by some amount. Sometimes it's not much, since the loading is actually CPU heavy (decompression), but sometimes it's disk speed limited. If it's disk speed limited, SSDs will decrease loading times by over 90% in most cases. This usually also includes your operating system. An SSD is not strictly necessary, but I'd recommend it.
2x8 versus 1x16 is nearly personal preference, but 2x8 is faster since it's dual channel. Note that most people are never really limited by RAM speed. The only game I can think of where RAM speed is an issue is Dwarf Fortress, so it probably won't come up. You also get the advantage that for $5 that it's almost guaranteed that both sticks of RAM will not be DOA, so if you're worried about getting your new computer up as quickly as possible that'd probably be a plus.
When buying a motherboard for price, the first question you should be asking is what features you need. Then buy the most expensive motherboard that has those features and nothing extra, and you'll get a decent mix of quality and price efficiency. What features on the MSI Z170A can you not live without? What features does the Gigabyte GA-B150M-DSH3 require? Why is the Gigabyte board trash? More so, do you actually require a non-trash motherboard? Even the worst motherboards these days are actually pretty decent overall.