There are a ton of Pactor modems for sale as most people are moving to satellite phones.
The latest version  is supposed to be twice as fast but a Pactor 3 will do the job.
I haven't seen the "ton" of Pactor modems for sale (there's only one used Pactor modem for sale on eBay at the moment....an older PTC-IIe). And, I hope folks haven't been deluded into thinking that sat phones are a good substitute for Pactor modems.
Yes, the P4 modems are twice as fast as the P3 modems, and many report that the P3 connects are also better/faster with a P4 modem.
As Dave said above, the Icom M600 can be made to work with digital modes, but it requires some modification of the radio itself. Further, the M600 suffers from a malady occasioned by potting of the trimmer capacitors which can cause overheating and shutoff. Not a great radio for email, for sure.