If you're looking to buy as few tools as possible, you might be better having a carbide tool, or just using a bowl gouge as previously mentioned, as long as they are sensible sizes for the project you are planning. All my stuff is normally pretty small, so the tools are quite interchangeable.
Not having a roughing gouge also limits the chance as a beginner of making the - usually pretty dire mistake - of trying to use it to rough down a bowl blank instead of a spindle... Look it up on YouTube, the results can be very dangerous. At least using a bowl gouge where it's "not intended" to be used - on spindles, doesn't carry the same risk of injury. In fact, one of the major authors (I think it's Keith Rowley) suggests they really should be called deep fluted and shallow fluted gouges, as they're useful for many things, whereas the roughing gouge should really be called the spindle roughing gouge, for safety's sake!
Nic.