1) The heating element is enclosed in the "metal part".
2) Since it is kept submerged I think the metal is pretty much always kept at a moderate temp - safe enough for the vacuum bags to rub up against without issue - though good practice would be to keep the bags back a ways from the unit to encourage good water flow. Water conducts the heat from the submerged element away from the unit pretty efficiently so the metal containment doesn't seem to get "really" hot.
3) How long the unit takes to heat your water depends greatly on the volume of water in the container you are cooking in. I don't know how big your container is but a smaller volume of water means faster time heating to the desired temperature. I use a big insulated cooler for most cooks, so I dump in hot water from a tea kettle during my preheat to help speed the process.
I hope these comments are helpful. The above are just my personal experiences and observations. Others may have something more or different to say or add.