This all depends on what kind and size of turbo you are selling and how much they want to boost it.
Wastegate is not needed if turbo already has an internal wastegate.
Blow-off Valve is not needed but recommended.
Intercooler is not needed but recommended.
You'll need an intake for it.
Charge-Pipes
Down-pipe
Fuel Injectors are not necessarily needed unless you're over 50% duty cycle. (I've seen duty cycles maxed out with stock injectors but they tend to not last long).
And a good Engine Management system if not just using an FMU.
Also, I suggest changing ignition timing... I forget if you advance or retard... I'm tired
Also, make sure the spark plugs are the right heat level for your application
There's a lot more you might need to have done, but like I said, it's all about the kind of turbo, size, and boost level.