Poker edge is not allowed on some sites (Pokerstars).
The price 69.95 for PT3 is lifetime for 2 licences (two PT3's running on 2 different computers with one serial).

I use PT3, Pokerstove (free) and MS Excel (I track my sit&go game there, even though PT has it built in, I like to organise it by myself).

And no they are not vital for success, but they can improve your game and winrate.