Powers get added together and toughnesses get added together
As for the best, really hard to say
If the collection is 99% modern commons with nothing exciting, sell locally or to a bulk buyer
(Some items hurt you, some are a mixed bag.) The game is extraordinarily difficult, requiring way more coordination and reflexes than I possess, and it is unforgiving: death sets you back to the beginning every time
A Ravenous Rats and a Megrim on legs is nasty, hence it being here, but just one foe, means it's less powerful than forcing everyone to discard, like Tinybones Joins Up, or Liliana's Specter
Special Summon 1 Level 6 or lower monster from your hand, or, if you control "Toy Box", you can Special Summon from your GY instead, except "Toy Tank"