I sold my RX8 because I'm always trying new things - that's how I could write my guide.
I've had two MMMs, an RX8 and an MMP.
Why do you only want to run 4S?
Use the 1900KV motor with 4S - it'll eke out a longer run time in finals. Also remember they do two lengths of motor - you need the longer one (for truggy use, extra torque over the buggy motors).
To get 15 minutes, you'd need one of the Hyperion VX 5500mAh 4S packs - IF you drive well/cleanly/efficiently. If you're not so sure, get the Hyperion VX 4S 6500mAh pack.
The slipperential IS the thing to have. I love mine. You'd need the 46T spur with the XT8. It helps you drive the car on loose tracks, saves the motor from harsh landings and bumpy tracks, and aids traction. It also can prevent you from going 'full beans' out of corners as it slips a little - stopping you from crashing into the next corner as you go too fast!
Mechanical brakes vs ESC brakes is an old argument now. I personally like ESC braking - I never used the extra tuning offered by mech brakes when I ran nitro, so I saw no need to retain the extra 100g for the servo, and the added complexity. Many are the same as me, or are happy to give up the adjustment. It's up to you. Personal preference. The RC-Monster kit is better suited to ESC brakes though - it'll need a little work (making servo mounts etc) for mech brakes to work.
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