The inlet connection on this machine is standard 3/4" threading, the outlet is 3/8" threading with a 3/8" quick connect fitting installed.
Yes, I would suggest at least 1 ft of rubber flex hose coming off of a hard pipe. I would not be concerned with the outlet, using the high pressure hose is sufficient.
This machine would need at least 50 psi and 4 gpm going into the machine in order for it to operate correctly. You would not be able to run this machine off of a well water supply.
Yes, this machine has a pressure relief valve on the pump but it does not come with a pressure gauge. You would have to purchase the pressure gauge seperately.