At the risk of sounding like a broken record...What are you trying to accomplish? You specified "Function Generator" but only talked about sine waves. What did you really mean??? Do you need triangles and squares?
If you need sine fidelity, use a triangle and diode shaper.
I set out to build a DDS function generator around a PIC processor. I was really impressed at first. Sine waves looked nice on the spectrum analyzer, even tho only 8-bit. But when I looked in time domain, I was horrified. There are some interesting transient anomalies, especially where the cycle length is one different from the number of stored samples. Yes, you can filter anything and get a sinewave, but at 5 Hz. that's gonna be some big filter. And triangles and square waves weren't even close to being what they should be. You can have sine wave points moving all around and filter 'em. Won't work for squares.
So, do you really need a whole function generator, or just sinewaves? Purity? Can you stand AC coupled output? Amplitude range? Noise tolerable? Harmonics vs noise vs FM?
The more you think about it the better you're gonna like triangles and diodes. The single-chip analog function generators should work fine at these frequencies. You can get reasonable (depending on your definition) repeatability with digitally controlled current sources. You're only asking for 500:1 range. mike