do i need to buy the factory roof rack before i can attach a bike rack on the roof?
In a word, yes.
You can buy aftermarket rails (will mount to the roof), risers (mount to the rails), and cross bars (connect the risers) however, you'll be into the rack for about $400 without even being able to mount the bike. From there you'll have to buy the bike rack(s) for as many bikes as you want to mount for ~$100 each.
My advice, depending on how tall you are, would be to get the hitch rack. I have one and it works really well. I personally have the Yakima Double Down, it works well due to the ability to lower it, with bikes attached, and open the rear hatch. My wife, 5' 9", about 130lbs, can load/unload the bikes with ease with this setup.
If you're taller, with some strong shoulders, you can purchase a factory roof rack from a junk yard for less than $100 typically. This will get you the rails, risers, and crossbars. From there you'll need the bike rack(s) for the number of bikes you have again, ~$100 each. My wife CAN NOT load/unload the bikes with this setup. I am 6', about 185lbs, not trying to brag about how freaking strong am I (yeah right =p) but can shoulder press about 225lbs. I can load/unload the bikes with this setup however, it's not the easiest thing. If I am taking more than just my bike, I will not mount them on the roof, unless I am hauling 4+ bikes, or going on a long trip.
For reference, I am talking about a Cannondale F7 MTB (wifes) and a Cannondale Trail SL1 MTB (mine). These are not light bikes. If you're riding a fully carbon road bike or something, shoot, throw it anywhere you please.
No. There are mounts that can attach to the vehicle without OEM racks (i.e. Yakima Q Towers) They can be rather spendy and you still need to hump the bike up there.
Receiver hitch is way more versatile for the money.
Check out the various rack manufacture websites for specific info.
While this is true, for a more seamless integration the Q towers are less than ideals.