I am currently restoring two Mobylettes, a 1961 AU85 (pictured) and a 1965 AU88. I just started both restos.
The AU85 I found in my backyard. The AU88 I purchased from a student in Paris. Both machines are complete.
How much time depends on what you need to do to the vehicle. I am doing only what is needed, I am not tearing down the engines, etc. unless absolutely necessary.
Your top expenses will probably be as follows:
Spares: try to buy a complete machine that has all the orginal parts. Paying and shipping for individual missing parts adds up.
Work done by others: don't be intimidated, try to do as much as possible yourself, just go slow on the tear-down and use a lot of liquid wrench (and time) so you don't break things.
Re-chroming is also hugely expensive.
Jump into it, you will learn a lot.