For what your planing on doing with this kit, I would check out this option!, it will fit a lot better in the chassis than a castle unit. https://www.hobbywingdirect.com/coll...14796877398131

Is this a necessary upgrade for the original eb410?:

If your going to need to run 2 fans just make sure the fans can run on your batterys voltage.. a Y harness for the 2 fans will do no harm . how much will then fans need and use?

Should I buy this for my original eb410?:

If your running an original eb410 aluminum shock caps are a must.

That esc and motor combo is more along the lines of a monster truck setup. Go with the 5700k for the eb410. It will be super fast. I had the 5700k in an hpi blitz and on 3s the front would not stay down at top speed.

The 3800kv (1410) motor was designed for the 4wd sct class and mod1 gears, it has a bigger rotor and so much torque that it will quickly destroy the eb410 driveline. You can reduce torque through the esc but you might run into gearing limitations. The 1406 series motors are more suited to 1/10 4wd buggies, they can be geared higher for better top speed and still won't break a sweat.