From my own personal experience (growing from "no-idea-what-a-transistor-is" to making a living from electronics design) I'd advise away from buying too much into the future. You might end up with more, and more expensive, equipment than you'll ever need. First let the need (and understanding of the requirements) present itself.
Typical DACs have clock frequencies in the 10-25 MHz range. Some more fancy ones are moving into the 50-100 MHz range though, and a DSP ahead of it might go to 100-400 MHz.
For basic toubleshooting a bandwidth of 4x the clock frequency should suffice. For more advanced situations (e.g. timing issues) you might want 10x. Proper probing of fast signals however might also require more specialized probes (e.g. active ones).
Personally I still consider a well-maintained general purpose analog scope (e.g. Tektronix 465, 2445 or 2465) to be an excellent choice for audio work. It probably covers 99% of your needs.
Samuel
