Neil,
Congratulations on your S40 and welcome! I'll offer a few thoughts that run counter to what's been suggested thus far, not because the suggestions are "wrong" but rather to give you ideas of things that work so you can decide whether or not they'll work for you.
I bought my S40 as a complete bike as set up by Cruzbike: 1x drivetrain, disc brakes, etc. I've since changed to 2x drivetrain and rim brakes.
Where I live (SW Wisconsin) it's relentlessly hilly and 7-10% climbs are the norm. I wanted more range at both ends of the gearing. I found I "spun out" too soon on descents and wanted easier gearing on the climbs. I went with an 11 speed Sram GX group with an 11x42 cassette and 50x34 compact crankset. I'm very happy with the setup and it shifts perfectly (this from someone who is rather obsessive about shift quality).
As mentioned, I also changed from disc to rim brakes. Although I'll fully agree that disc brakes provide more stopping power they also come with a weight penalty and I was looking to reduce the weight of the bike as much as possible given the terrain around here. As you no doubt know, rim brakes were the norm for bicycles for decades and work very well indeed. The topic of "rim vs disc" tends to be a fairly religious one in cycling so suffice to say that I'm happy with my choice and yes, I do have disc brakes on other bikes so I'm not "anti-disc" by any means. As others have mentioned, for the S40 you will need long reach brakes. I use the Tektro R559 set and am happy with them.
Finally, you mentioned your friend had a bunch of Campy parts and wondered whether those would work. They certainly will but you'll be more limited in cassette choices, road shifters will only work ergonomically with some handlebars, etc. The reason most people go with Shimano/Sram drivetrains is the availability of wider range cassettes and derailleurs that will handle them, more choice in shifters (road, mountain, click shift, grip shift, ...) but that doesn't mean they're the only thing that will "work".
Figuring out a combination that works for the style of riding you want to do, the terrain you ride on and will be comfortable ergonomically is a real challenge. I know that I came to recumbents from decades of riding on a DF and doing all of my own bike work and it was still a challenge. You'll find lots of helpful advice here and of course we all have our own opinions about what's "best". For the most part, this means advice you'll get will be from someone who has tried it and knows it works so you'll only have to decide if it works for you.
Good luck!
