STOP right there!
First, RC-Monster are releasing a complete conversion kit for the XT8-09 (or the older one with a new rear centre shaft) - which will be online this week. It comprises a complete chassis, motor mount, battery box and centre diff mounts.
Link. Note that the pictures there show the prototype - go to the third page and you'll see the pre-anodised final version.
Here's a sneak peek:
Second, I would go with the MMM along with a 2200 motor for 4S.
I'm not sure what Jason is talking about with the 'higher capacity ESC' with the Tekin - they are both rated the same, and from my experience the MMM will take more of a beating (excluding the fan, which is weak at best).
If you want to run higher voltage - go with the Tekin, 1700KV for 5S, and 1350KV for 6S. Make sure you get it from someone like RC-Monster - they made sure they got the new stock from Tekin. (Even sent old stock back to get them updated with the new rotor).
Maxamps packs...
Avoid.
Go with the Hyperion VX series for less money, and get more performance, better cells and better matching.
No chance will you get 20 minutes with a 4S 6500, you'll need closer to 8000mAh for 20 minutes with a truggy.
I can get 20 minutes (racing plus cool down laps) with a 5S 5500 pack, and I'd suggest that 5S is the voltage you'd want to run. Higher voltage combined with a lower KV motor will mean cooler running, and thus higher efficiency than a higher KV motor, with less voltage, as there would be higher current flow.
Try reading
my beginner's guide, as it will answer many of your queations.