Maxing out the RAM and especially a large, modern SSD are certainly options to consider.
However, the way you described your setup there will be probably quite a bit of cruft accumulated, caused by an upgrade over an existing installation and/or Migration Assistant afterwards.
Before throwing any money at it, you should try how the machine performs with a clean install. Given that you are still on rotating magnetic disks, any external drive to try this approach should not be so drastically different and give you an impression of the changes achievable.
That is radical, takes a while but is also easy. You may also try to analyse, what really causes the slowness in your current setup.
If the perceived slowness is in Sierra but not in El Capitan then it stands to reason that there is too much going on behind the scenes.
Fresh installs of 10.11 vs 10.12 are not that different in terms of perceived speed on this kind of hardware.
If you are successful with improvements on you current setup and want to keep the hardware: then by all means necessary, go an get an SSD (installed).