I had an external wastegate on my old S14a.
They dont make any chatter noise, they just vent the excess wastegate gasses to atmosphere rather than down the exhaust and are not any kind of replacement for a dump valve - they do different jobs.
To fit it you would need to buy a manifold with the fitment for your turbo and the external gate adapter as modifying std manifolds just doesnt offer the flow characteristics.
You would also need to weld the std internal wastegate shut to stop that working.
External gates are a high power mod and you would see no power gain from fitting it on a std turbo.
Theyn are also a pain in the backside to set up correctly, took me weeks and weeks to get the settings right to get an accurate boost setting