"Also what is the preferred method of communication, is it Channel 16 or 121.5? "
I can only speak for part of the UK here ;
If the distressed vessel is in within marine VHF range of land then the coordinator will probably run everything on Marine VHF Channel 16 (156.800MHz) if it is a serious incident. (And by using the proword "Seelonce Mayday" tell everyone else to refrain from transmitting or "shut the f+++ up".)
The SAR Mission Coordinator may move comms to a working marine VHF channel, if it is a less serious event.
"Finally, is there any particular advice or recommendations for large vessel crew/captains performing Medevacs which isn't specificially in the IAMSAR book?"
Again this varies from country to country. In UK waters you can expect to be connected to a doctor by marine VHF to discuss the symptoms/injury and severity before a decision is made to medevac. Outside of coastal waters Inmarsat is often used for this.
I understand the USCG for example always use a rescue cage (may be wrong here) while UK helicopters will use a single strop, a double strop or one of a variety of stretchers as the case demands.
Since you seem to be asking as a Master or potential Master - The answer is that the coordinator will expect you to begin on marine VHF16 and then direct you to stay on that channel or use another, while the helicopter crew will select the place on the vessel they think is best and use the method they want to.
It all boils down to "the people doing the extraction will tell you what they want". This will probably involve steaming on the course they choose directly in into wind or at their preferred angle.
A significant point here is that the Master will be expected to conform to the "requests" of the rescuer.