If you dont have a servo tester or some other way to power on the servo you can always go ahead and just attach the servo horn now and then come back and readjust it later after you have the electrical system all soldered up and installed.
As for the pinion. Pick one in the middle and try that...
If it calls for +5 and you only have +3 then yes the wheels are technically 2mm to close to the chassis. Getting hexes that are +2 Will push those wheels out to where they should be although its possible they may work anyway. You can always fit them and see if the wheels hit anything throughout...
The body you need to compare the wheelbase of the body to the wheelbase of the car, Ideally compare the width too but the wheelbase is the more important thing there.
Not sure what to tell you about the wheels. I've only just started looking into drift kits myself so I'm also still learning about...