About 500,000mph
The link is detailing plasma thrusters. They use the thrusters I suggest using for the craft.
Nothing to do with the combustion temps. More as to do with the plasma generated and the thrust generated.
Magnets do not decrease mass but they can be used to create a relative weightlessness to an object. Basically the magnets are used to generate resistances to one another generating a relative hovering ability.
It would take energy to do this yes but with enough constant electricity being generated it would be feasible. This would be essential to continuously have fuel and water needed for the trip.
It may seem less efficient but in space one must weigh the pros and cons to meet survival needs. This would need to be done to continuously travel and so would stopping to gather needed resources. Because nothing last forever.