Getting a chance to test a character before you buy him would be great. Especially for newer players, who don’t even know the abilities of a specific character. They would just end up buying the one that looks the coolest.
My suggestion is a Trial/Renting system. You spend like 1/5th of the full price for a character and then you will have him unlocked for one week (or so).
The first few days/weeks are the most important ones when we’re trying to get new players to play the game and keep playing it. A renting system would allow exactly that, by giving (teasing) them more characters early on, without taking away the grinding aspect later when you want to fully unlock a character after the trial.