Even with the radio(s) and antenna(s) that you will collect along the way, you will always find that you will need an extra piece of gear to help you get things done the way you want to. Here is a listing of what I have used along the way.
MODE J Filter (CF-4160J) - This is something that you will need to aquire if you want to operate full duplex on the FM birds. Without going into a lot of detail, if you wont have this on the UHF receive side, you will run the risk of desensing the UHF downlink and you wont be able to hear yourself. What you will hear is something that sounds like Donald Duck talking if you can even make out that much. There are several good links that show you how to hook this up. If you are using preamps such as the ICOM's that get power off of the coax, you will want to put this on the antenna side of the preamp to keep from blocking power to the preamp. I connect the common side of the CF-4160 to the Antenna connection of the ICOM preamp and connect the UHF labeled connector of the Diamond duplexer to the antenna I am using.
VHF/UHF preamps - When I am operating either portable or mobile, I dont have my ICOM IC-910H setup from home to use. Depending on the satellite I am working and the conditions I am having to deal with, being able to boost the receive signal a little bit to make it easier to hear is nice. Advanced Receiver Research is a nice, small form factor preamp that is easy to pack and take with you. You do have to be careful with certain models that you dont transmit into them. If you do transmit into a preamp, you can expect to fry it and have to buy another one. Some of the more expensive models do have the ability to either put themselves into bypass or otherwise protect from the transmitter power coming in over the coax.
Audio Processing - Being able to "clean up" the received audio isnt a problem on the FM birds but could come in handy on the SSB birds since you can have multiple users on the same bird at the same time. I dont use this device a lot but it is handy when those times do come.