Just caught up with this thread.
I'd suggest that whatever concoctions you have in your bottle, it's worth having another one with plain water, just so you have the option. It's also useful for things like pouring on your head when it's hot, rinsing foreign objects out of eyes and cleaning grazes.
Mudguards are, in general, worth the pain.
Clipless pedals is more of a personal thing (though the experience of falling off in embarrassing
[1] circumstances once you think you've got the hang of them is almost universal). I find them useful for keeping my knees straight (and as a safety feature on recumbents), but flat pedals and decent shoes can work just fine for many people.
As for brakes, single moving pad is standard for cable-operated discs. I previously had a set of cheap Tektros that were capable enough at braking, but required constant tweaking. Received wisdom seems to be that the Avid BB7 is the best of this type, and I've found them to be reliable and relatively easy to tweak. There's a TRP mechanical brake that moves both pads at the same time which some people swear by and others swear at. Otherwise, hydraulic brakes move both pads as standard, are lovely to use and tend not to require fiddling-with. The main advantage of discs over decent rim brakes (eg. well-adjusted V-brakes) is that they work consistently in the wet, the braking power isn't inherently better.
[1] Falling over in slow motion in front of an audience at a pedestrian crossing is traditional, but I went for the underrated pratfall-into-a-bed-of-stinging-nettles option.
