If you are really wanting to do a brushless setup, you should look into either a dual brushless motor setup on your 17.5 motors, kind of like the dual velieon setups that are out there.
Go with a single motor setup and an ESC that can handle the load on 4S. You could do the Novak HV systems which should work well. I run right now a Mamba Monster ESC and a Neu 1512 2.5D motor and its great.
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< Durango DESC410V2, DEX410V5>