For the flux wire - you can use fluxed wire with any mig, but for good results you'd need to change the polarity so that the gun is negative and the earth positive. A lot of gas/gasless welders have terminals on the front that can be switched, and others have terminals inside the case. You'd need the manual really.
No idea about gas pressure, although a flow rate of about 10 litres a minute would be a reasonable starting point.