First of all when you download the macOS 10.12 Sierra installer, you must run the .pkg "installer" app which will only extract the real installer to the Applications folder. After running the .pkg file from the mounted .dmg file you need to run the real installer "Install macOS Sierra" from within the Applications folder.
Instead of installing macOS 10.12 Sierra, you should just skip straight to the macOS 10.13 High Sierra installer:
How to download macOS - Apple Support
The link above also has the instructions for how the macOS 10.11 & 10.12 installers work. If you are installing a new SSD, then I highly recommend creating a bootable macOS 10.13 USB installer since you don't have access to Internet Recovery Mode (Command + Option + R) yet since Internet Recovery Mode gets added with a firmware update with macOS 10.12.4 (or .6).
Create a bootable installer for macOS - Apple Support
If you are upgrading the current drive, then make sure to have a good backup just in case something goes wrong.
As @John Galt mentions the SSD upgrade will provide the biggest performance boost for this laptop even if you stay with the 4GB of memory. Plus the SSD can be re-purposed as external storage once you retire this laptop, unlike the memory which won't have any value after retiring the laptop.
If, however, you discover the memory is also a bottleneck after the SSD upgrade, then you can decide whether it would be worthwhile. To determine if memory is a bottleneck, you would use Activity Monitor to monitor the memory usage. As long as the memory pressure is green and there is not GBs of Swap or Compressed memory, then memory is not a bottleneck. If memory has not been a problem before, then I doubt it is an issue now and is not worth the cost. If you believe otherwise, then just purchase a single RAM module to pair it with one of the original 2GB modules for a total of 6GB or 10GB.