Buying the Phenomenal Character Pack, which guarantees you a full character unlock every time, is definitely the fastest way to build your roster of the basic characters; just bear in mind that you won't get Legendary characters from that pack nor will you receive the specific characters released with the various DLC updates.
As for the fastest way to collect PvZ coins; I would suggest doing Garden and Graveyard Ops or if you're Multiplayer oriented go for Turf Takeover. Your total coin award from either is largely performance based, more so for Multiplayer, but on average you should be able to collect 20-30,000 PvZ coins from a really good full game of either.
