I would also recommend going with a Druid. In your original posting, you said that you have access to nearly any 3rd ed book. If you want (and the DM allows it) try going with the Shifter PrC from Masters of the Wild. It is an earlier edition of the Master of Many Forms PrC with more options of things you can change into. With 10 levels of that you can change into any size creature (except Colossus), and any type of creature (except Fey), at will. For your last few levels, take 5 levels of Warshaper (I believe it is in Complete Adventurer). With this option, you are severly hampering your spellcasting capabilities, but you gain the flexibility to be whatever is needed to complete the task at hand.