i will buy a light side and a dark side deck from someone(must contain good cards)

Your best bet would to be to buy a box of Reflections and/or Reflections 2, those sets' packs contain 18 cards each- Reflections packs have one rare card that's been foilized and 17 randomly sorted cards from all previous sets (up to Jabba's Palace) and typically contain 2 or 3 non-foil rare cards. Reflections 2 packs have 1 foilized rare, 2 (or 3, I forget offhand) cards from the Expanded Universe (Prince Xizor, Thrawn, etc), a combo-card (combining 2 previous interrupt cards into 1 card) and the rest are randomized cards that typically include 2 or 3 rares (up to Death Star 2) and sometimes have a japanese version of a card.

Getting a box of one or both of those expansions will give you a good deal of cards from all the sets and give you a pretty solid base-selection to make a deck out of.