Will this fit my 2018 Softail Slim?

It is listed for the FLSL Softail Slim 107, so it should fit that model. Confirm your exact year and engine before ordering, since fitment is listed as specific rather than universal.

Is this an OEM Harley starter?

No. It is an aftermarket starter from Rareelectrical, not a Harley-Davidson part. Expect aftermarket build quality and check the warranty before buying.

Does it rotate clockwise like the stock starter?

Yes, it is listed as 12V clockwise rotation, which matches the stock direction on these models.

What is included in the box?

The listing does not state box contents. Assume the starter only and plan to reuse your existing mounting bolts and wiring unless the seller confirms otherwise.

Is it cheaper than a Harley OEM starter?

Aftermarket starters usually cost less than OEM, but no price is available for this listing, so compare against a dealer quote before deciding.

Can I install it myself?

Yes, if you are comfortable with basic wrenching. The starter bolts to the transmission case and connects to the battery cable and solenoid wire; a service manual helps with torque specs.