You didn't say what exactly the Terratec/Ableton setup involves, but since Terratec manufacture audio interfaces, maybe you already have an interface?
If you want to record guitar direct, you probably should get an interface with a high impedance "instrument" input (unless you use a separate DI box), for recording from the line out on your amp you just need a line input. Most of the interfaces on the market should come with Mac drivers or use built in drivers and not need any extra software.
Garageband will have drum sample modules and various amp simulations - they won't be up to the same standard as a dedicated application, but should be enough to get started with.