You HAVE to be able to supply 48V into a 2 ohm load because that's what the system sees when it starts. t=0+. This is also the locked rotor current I=48/2 Now, i don't know how long you have to be able to sustain it, but that can be determined by any protection mechanism you might employ. The simplest would be a little larger than the expected blow time of a fuse.
Any FETS selected for the bridge should be based on SOA or Safe Operating Area. Pulse response will be there too.
Again, the surge ratings of the bridge and the power supply also has to deliver the voltage for longer than this time period.