Hey man... welcome to your newest addiction. As far as rebuild, a well cared for VG with 103k miles is far from needing a rebuild. But if you are concerned, do a compression and leak down test to see what kind of shape the upper end of the motor is in. I would have the 120k service done, and check all the connections on the harness for corrosion, and add some dialectic grease when reconnect everything. 400Hp is pretty easily attainable with bigger injectors, 550's-740's, a boost controller, and a good tune. Good luck.
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