Just go with STI and/or Ergo levers for shifting - forget the bar ends, brifters are fine, shift flawlessly, and allow you to keep your hands where they belong. Shimano is prone to collect dirt and it effects shifting, but really only in extreme conditions. Campy’s design is better in this regard, but really it’s what you’re comfortable with. Unless you want the bling, go with 105, or Centaur level stuff. Works great, cheaper to replace (and you will be replacing components).
Whether or not you go with a single ring up front depends solely on your strength as a cyclist, and whether your racing or just riding around off-road. I’d recommend going with double rings. The one thing I’ve found to be true with amatuer 'crossers is that virtually everyone is over-geared. There are very few people that need anything larger than a 42 up front, and most need a 36, or even a 34 inner ring to get them up some of the steeper stuff. Cracks me up to see people struggling to turn the 46’s, 48’s, etc., there’s just no need unless your spinning out of those gears routinely.
Long-cage derailleurs are a good idea because it gives you more choices for gearing in back.
Top-mount brake levers are a matter of personal preference, if you’ve done a lot of mountain biking, you’ll probably like them, but the cheapers ones do provide weaker braking. Go with quality here.
Brakes? Depends on what you want to spend. Paul’s are hands-down the best, but expensive. Everybody makes a big deal about clearance with the neo-retro’s in back, but that all depends on your size. If you ride a frame smaller than, say, a 54, you might hit your heals on them, but otherwise they’re great. Avids are hit and miss, Shimano actually makes a decent cheap pair, and then there are the “Euro” models that are available from Cyclocrossworld.com, but I’ve tried nearly all, and the Paul’s work best, and are amazingly easy to maintain.
Wheels are where good decision-making will save you time and money. First of all, tubulars make a difference in cross. Whichever side of the tubular/clincher debate you’re on, everyone agrees that tubulars and cross are like PB&J - they just go together. Tubulars allow you to run lower tire pressures, which assures you of better traction without the worry of pinch flats. As has been stated, Tufo’s make some great tires - and they’re tubeless so pinch flats are even less of a problem. Don’t worry about boutique wheels unless you’re racing at a high level, and perhaps have some sponsorship to replace them cheaply. Plain old 32 spoke Open Pro rims work flawlessly with Shimano or Campy hubs. Go with wheels that are easily maintained. If you must go clincher, Tufo’s clincher-tubular will work well - some might argue better because they roll off less. I’ve found this to be true, but believe it’s due to poor gluing of the tubies.
Wipperman stainless steel chains are a must.
Pedals. Time ATAC’s or Crank Brother’s Eggbeaters (Candy’s or standard, doesn’t matter much) are your two choices. Shimano’s top-end mtb pedals will work, but they clog quicker than the Time’s or CB’s.
Have fun, if you’re going to race, practice your dismounts and re-mounts until you’re very, very confident. Doing them in practice as opposed to during a race is night and day.