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#11
I usually buy all the parts at the same time.
It doesn't make sense to buy a bit at a time since there may be sales on the parts you already bought while waiting to buy the rest.
Also, with Diablo 3, playing with a friend who is 5-10 levels higher than you will get you equipment drops for your current level or a level or two higher.
Was a level 35 demon hunter last night playing with a friend who has a level 47 barbarian, played for about 4 hours till I hit level 42 and always had current level equipment.
You still want to shop the auction house for weapons and specific equipment since you can find sweet deals there.
I needed some gems so I bought 10 helmets for about 1k gold each that had a gem installed, removed the gems and just sold he helmets.
There were no gems coming up in the auction house at the time so I just bought the above.