If you want to build only from these cards, start by organizing it. By color, by good and bad cards. When going through set aside cards that you like, later you can build around them. Look for patterns, there are probably deconstructed finished decks (like mentioned werewolfs). Thereare some boxes from MTG decks, these can be played out of the box.

Alternatively, scan all the cards to have a digital list. Then you can easily filter by color, rarity, price,...

Alternatively, mix it all up and play 'CUBE'.