Seat fore/aft position should be all about getting the proper knee angle, not abused to make up for an improperly short or long top tube. Different stems and front bars will help with frame fit to make finer reach adjustments.

And as for the disc brakes, I've ridden my brother's mtn bike that had them and while they are nice, I wouldn't exactly claim that they're necessary. All I noticed was that it was a lot easier to lock up the tires and skid with the brakes fully applied - you're limited by tire traction. It might help on extremely sharp descents with heavy weight. But still, plenty of 300-400+ lbs people are out there using conventional brakes.