It's a LOT to take in...I've been a regular of these forums for two years & I'm still awaiting advice on mid-level brakes on another thread.
You'll use your rear derailleur a lot more than the front because you'll have anywhere fron 7-10 gears back there that are closer in ratio, so that's a perfect example. Same with brakes...the front gets 75-80% of braking power so you'll use it a lot more than the rear. However, I find that it's much easier to get the hang of proper distribution of braking power if your brakes are the same front & rear (unless you're a really skilled rider, in which case many people will have larger front rotors than rear).
But really the one thing you don't want to skimp out on is the frame. Everything else can be upgraded piece by piece if you decide to hold onto the same bike, but replacing the frame is much more costly & difficult. Next to that I would personally go with the quality of the wheels because I've had bad experiences with cheap wheels going out of true. Then fork (some may put this ahead of wheels), brakes, derailleurs, seat, crankset, handlebars/stem, chain in that order for me. Others will probably rank importance somewhat or even very differently, but I think everyone will agree that the frame is the most important.