Can buffing and sanding attachments be used with a fixed speed powerdrill?
The answer is yes, but not as effectively as a purposefully designed tool. Take a look at buffing and sanding tools' RPM. It's significantly higher than that of a power drill: 8,000 - 15,000 for an air powered palm sander vs 300-1,500 RPM for a standard variable speed drill.

Other aspects depend upon the modular design of the particular tool you're interested in. For example, a simple rotary sander will leave circular scratches in wood that a random orbital sander will not. These scratches become very evident when stain is applied.

Power is another factor. Purpose-built tools are designed to provide peak power in their application range. A multi-purpose do-it-all tool will likely compromise this "tuning" to be able to work in all applications.

If I'm understanding your question correctly, tool kits like this do exist outside of the gimmicky "As-Seen-On-TV" ads. The Ridgid Jobmax is one example, and the Black &amp; Decker Matrix is another.