If you buy a complete without setting it up yourself use those bearings first. Most come with a decent abec 5 bearing or something similar. Pick up some Bones Reds they are inexpensive but are fast. Check out Rat Vision on YouTube or on here they have a great video on cleaning. Most bearings can last a year or so if you clean em and lube em. It does depend on how much you ride though and where for that matter.