That power supply will work fine. I don't think the back emf is too much of a problem with stepper motors. The gecko doesn't self destruct until over 60V so I don't think you would have a problem even if it went a bit above 50V for a really short time. This is a switching power supply so it does not have big capacitors like linear supplies. I am not sure how well switching supplies absorb back emf anyway. One advantage is that most power supplies like this have a pot to adjust the voltage up or down by several volts, so you could turn it down a bit to be safe.
You could build a circuit to cut in a resistor to burn off excess voltage but it is really not necessary at all for a project like this.
The Keling supply should work fine, however, I like linear supplies myself. I buy mine from www.toroid-transformer.com.
The keling supply does not have any other votlage output, just 48V.
The fuse should be sized 20-50% larger than the peak current draw for full load. I am guessing 3-4A. It will say on the supply how much AC current it draws.
I like linear supplies because they are capable of short period overloads. I think that switching supplies automatically limit the current at its rated amount. I am not sure how it will react if there were a short on the DC side. Maybe someone who knows more can chime in on that.