Disc brakes rock. Comparing the v-brakes on you bmx bike to a good set of discs on an mtb isn't going to get you anywhere. Two different bikes seeing very different kinds of riding, if used for their intended purposes.

Hydro vs. mechanical is a can of worms. Hydro? Many, many options. My preference. Mech? Avid BB7 are the best, by a long shot.

If you're still looking at buying a bike and upgrading everything, save a bit more cash to get the nicer built bike to start with. You'll end up paying less money in the long run. Or buy what you can now, don't worry about upgrading until something breaks. Then replace the broken crap with nicer.

And yeah: dude. Come on. Doing a bit of searching isn't too much to ask. All the info is out there. In the time spent starting all these threads and replying to various answers, you could have found the info on your own, pretty easily.

It's not rocket surgery.