With your goal to lighten the buggy, I wonder if a 36-50 motor woundn't be enough. Since your not going for incredible top speeds, or crazy long runtimes a 6s seems overkill to me as well.

Your plan sounds fine but seems contradictory to your goal.

I have a heavy mostly alloy HPI NMT(very close in size to an 1/8scale buggy) that does close to 50mph for 10 min on a Hacker C40-8L and 4s A123's.



My Emaxx does 42MPH through grass for close to 10min on 5s A123's and MR36-50-3300v2



I think you can meet your performance goals and reduce weight even further!