The short story is you need 100 Mb/s of real IPS sensor bandwidth (50 Mb/s x 2 for each direction of transmission). Cisco's sensor specifications are overly optimistic by a factor of 3, so that in Cisco speak, you are looking for a 300 Mb/s sensor. You appliance choices are:

4240 - 250 Mb/s (maybe you're not going to use all 50 Mb/s of the WAN link)

4255 - 500 Mb/s