You can install any modern graphics card you want if you have an adequate psu.
pcie 2/3 are forward and backwards compatible, so no issue there.
Here is a nice chart:
http://www.realhardtechx.com/index_archivos/Page362.htm
I might suggest a GTX1050ti would be appropriate.
It needs minimal power, but you might consider something in the 550w range to allow for a future graphics upgrade.
Quality counts, try to buy a tier 1 or 2 unit from a list such as this:
https://linustechtips.com/main/topic/631048-psu-tier-list-updated/
A SSD is a wonderful performance upgrade; it makes everything quicker.
I like the Samsung evo devices.
Do not worry about sata 2/3; most of the benefit of a ssd is in random I/O.
A ssd will be perhaps 50x faster in...