I believe the 7.5 is around a 12 - 14 brushed mod motor. But don't just think speed, you need to think control as well. Lots of guys were going for all out speed getting 5.5s and were selling the motors to get a 6.5 or 7.5 to have more control. If you really though can control the power, maybe start with a 6.5 . But I think 7.5 would be good.
As for the differences between the two types, the Factory team is full graphite, has titanium turnbuckles, lightened diff outdrives, threaded shocks, etc. Its fully options. Only thing ironically people change out is going to the plastic chassis.
Keep in mind graphite is stiffer and will break easier in hard crashes. Plastic will give more.
If you went only with the regular b4 say used, make sure it has the following:
1. Aluminum Front hingepin brace
2. Titanium turnbuckles ( prefer lunsford over AE brand)
Thats it that I would get right away.
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