if you intend to race then definitely get the SP kit. you could always get the original ZX5 shell and fit it if you prefer the look of that shell, although it is very tight to fit electrics under, the new shape is a little more roomy.
The SP is also far more competitive for racing than the RTR, unless you spend a fair bit upgrading the RTR which will make it more expensive than the SP anyway.
In terms of motor. This depends on your track and driving ability. If it a reasonably large track and you are a competent club racer (from a different class) then either a 5.5 or 6.5 should be ok.
If you are a complete novice then maybe something like a 10.5 to begin which you can later upgrade if the speedo will take a lower wind motor.
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