Hello, just signed up myself and your post caught my eye as I also have an 8.6 DS, could not be any happier with it.
For a computer, I got the Sigma BC 12.12 STS as I feel that cadence is too much information, especially as you get to know your bike.
For pedals, I started with Shimano PD-M324 dual platforms as it was my first experience with SPD. Found the dual-sided flexibility a help
as I got accustomed to SPD, but soon found myself staying clipped in at all times and got tired of fumbling around to find the right side of the pedal.
I have since upgraded to Shimano XTR-PD M985 and love them. Pricey but worth it in my opinion.
For a seat, I would not change. Hope you enjoy your new bike. Here (I hope) is a picture of mine: