I am using an 8gb pi and it’s using about 25% of ram with Mqtt, motioneye, DOODS, advised etc. I have read somewhere that 4gb pi4 is plenty for home assistant + add-ons. For the SSD I recommend buying an SSD and a usb 3 to SATA adapter because there are some compatibility issues going around with pi4s and SSD adapters. You can see more about it here. Installing Home Assistant on a RPi 4b 8GB with SSD boot
A 128gb SSD is the most cheapest and reliable as far as I know