The best thing you can do is learn how to shoot
mostly on the Nikon 50mm. Learning to move the camera instead of just use more glass makes for better photos. (its also your fastest (meaning lets in the most light) lens)
Ektar is really forgiving, and you an over or under expose it by a couple stops and still get decent pictures.
And yes, for nature in good light, 100 is more than enough, consider that most of the classic Nat Geo stuff was shot on Kodachrome ISO 64 (or slower) film.
I'd also tell you that one of the things that looks really really cool on film and is much harder to do without a much more expensive digital camera is long exposure low light stuff (IE, stop the lens all the way down to its smallest setting, and expose for 30 seconds or longer) - but it requires a Tripod. https://leho.blastpuppy.com/~aloha/photos/carls.jpg is an example of this technique.
You can get a better idea of the look, breadth and capability of film by taking a peek at my Gallery - https://leho.blastpuppy.com/~aloha/photos/
Film to me is much easier to make good pictures with - because other than light exposure correction, what you get is what you get, and the color profile and many other details are decided by just what film you pick to expose.
Its a matter of opinion, but I dont think any (currently produced) slide film looks better than negative films do, Kodachrome is the exception, and if it was available again, I'd be looking for another EOS-1 to go shoot it with.
Also yes I'm a Canon guy, but like, its personal preference really - and the first SLR I shot with was a Cannon AE-1, and I really like the Cannon EOS series cameras. I'd also strongly suggest shooting in Program mode - the Zone System was great for the B&W file when introduced, but it doesnt really make for great color photos - and frankly the engineers at Nikon or Canon are smarter than you or I, and you'll get fantastic looking pictures on Program Mode, allowing you, the photographer to focus on the thing that takes real artistry - image composition.