max038 function generator project

hello everyone
i'm planing on making a digitally controlled function generator based on the max038 ic
i want to use arduino mega 2560 to control everything (frequency, duty, waveform and amplitude)
i'm not entirely newbie but i'm not nearly knowledgeable enough to pull this off by myself and i know that i'm aiming high
but hopefully with your help i can do it

i want to base the frequency control on the design on page 15 of the datasheet:
http://www.phy.davidson.edu/instrume.../MAX038%20.pdf

like this guy did:
Frequency synthesizer with waveform generator MAX038

but i don't like the frequency range on this design 8khz-16Mhz in 1khz steps
i really want it to be 1hz steps (or less if possible) and start at as low frequency as possible

this project is about the learning process just as much as about the end result
so please help me do this

i want to start off with a question:
why did i see that in all the manually controlled project cosc was changed in order to determine frequency range
but in the pll controlled design like in the data sheet cosc stays unchanged
project examples:
High Frequency Function Generator with the Maxim MAX038
20MHz Function Generator
functionswp



and if i made a mistake and didn't choose the right forum please point me in the direction of the right one