hey colcapp,
i'm quite new in PD as well, i also started with the project fm synthesizer as my first real project and nearly finished it today.
I started with building some basic stuff, which i would need in many cases and patches, for example a simple ADSR abstraction or simple Synthesizer, with possible ADSR function.
This was quite quick finished, so i started to do the things about which i thought this are the most important. and after and after the other stuff came, so now i think its nearly a finished synth.
This is the way i can recommand.
If the ADSR envelope is important then start with this. if its the filter, start with this.
but i think what is important is that you look how you want your finished synthie to work like. so you should keep in mind what you want to add and make your patch tidy and open for new stuff.
I think this is an easy way to build a synth from which you have a little idea how it is working. And your not going to lose the motivation cause what your doing seems important to you.
jones