Well it al depends on what you want to spend. I have got all Systema parts in my gun except where I somthing better was needed, like Phoenix gears. But then again I spent a ton on all my stuff. If you want an all around great built gun I would go with all Systema internals. Look at the link below to see what I have in my build and then you might know what all you need for yours. But deffinetly do some price searching for a few days once you find exactly what you want. eBay is a good place, and the member eHobbyAisa has lots of Systema stuff at good prices. I am a Systema nut... others members may tell you to get other stuff, its all up to you, your needs and your pocket change. Heres the link to what I have for you to see what is generaly needed in a build.
http://www.airsoftcanada.com/showthr...611#post572611