1. Depends really on whether the vehcile has a trans or diff to start with. With a center diff, you can tune the car more so that it doesnt wheelie like crazy- better cornering too. Beyond that, there is the obvious weight saving, lower CoG usually, and you have fewer gears to worry about grinding compared to with a tranny.
2. Depends on how much room you have for cells to start with. Typically, you will see a 4s lipo setup using a ~2000kv motor, and a 6s lipo setup using a ~1600kv motor (assuming this is for racing rather than bashing really). The castle 2650kv motor is great for 4s lipo, but with a kv that high you wouldnt run higher voltage, but would run lower gearing perhaps (Im not the greatest expert on buggies and truggies).
3. Series harness- see BrianG's website for diagrams:
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