It's usually recommended to buy RAM in pairs although not absolutely necessary. One scenariuo where you might buy a single stick of RAM is if you plan on buying another identical stick when you can afford it.
You can just use that SSD on the board.
If you mainly browse, i5 is good, or Ryzen 5.
16 Gb RAM is good but 8 Gb is enough if you want to defer purchasing more RAM later.
Drives are cheap, look at buying a smallish nvme at around 512 Gb. Use external drives for extra storage.
I recommend buying a new power supply as well since yours is old.