Personally I would build it bit by bit as you have things to make noise and you will learn way more by building it bit by bit. It will also lead you down some weird alleys if you go to the second hand market as you will stumble across makers and modules that will change the way you think about what your doing.

It really depends on what sort of sounds your aiming to get out of it but I found doing it bit by bit I learnt so much more and it stopped me from getting overwhelmed with information.

You should also look at the 3 module series on youtube to get an idea of what can be achieved with a small system.