The benefit of adding to a semi-modular synth is that you've already got a solid base to start from. In this sense, you're not too limited by order of module purchase. The main thing I would say is to just focus on what you want the most in here and then space out the voice/effects purchases as much as possible so you've always got a good balance of sounds to modulation/utilities. Pam's is an obvious pick for one of the first modules, since it is a compact way to explore a lot of this stuff and will help organize future purchases. Same with the Quad VCA, which you will find immediate use for (especially as a way of manipulating modulation from Pam's in a tactice way).