This computer costs about $600.00 (check prices)
Maximum RAM: about $100 for 16GB
Hard Drive Upgrade: Not needed really unless you need more space. I would be fine with this size.
It’s quite possible you could buy this iMac for about $600 and then just use it. If you needed more ram you could add 16GB for $100 so about $600 or so would get you a very nice iMac with a fast i5 processor and enough ram and a fast SSD already installed