I mean I can't say for certain that it wont hold up to 6S power but Id prolly spend some time testing the waters with 4S first and see what happens. Those gears should be metal but were originally designed for nitro so Id test the waters first. It will be stupid fast on 4S anyway so unless you're...
the 6S Arrma stuff is good. Not to crazy about their smaller 3S and 4S kits. Kyosho is good, Team Associated, Losi, Mugen, Serpent, Xray, all make great buggies
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Id do something a long the lines of what Baker said and just put a Hyper .21 engine in that buggy and then just buy a brushless RTR if thats something you really wanted.
What Brushless RTR are you looking at?