I keep four different things in mind when I chose what modules to buy, in no particular order:
1: Budget. I maintain a list of modules I want and I kept an eye out for sales, marketplace , Reverb, Ebay, etc. When I come across a good deal on a module I know I will use I try and jump on the good price if I can.
2: Rarity. Some modules are hard to get because they aren't made anymore. If you want one of of those you're at the mercy of the market. When one pops up for sale that might be a rare opportunity to get it when perhaps later on you might not be able to find one for sale at any price. On the other hand, many rare modules are overpriced, sometimes very much so. So this is a balancing act. Overspending on rare modules is a poor idea, especially if you are new. But if you find a rare module that you know you will use and it's a great price? That is something to jump on even if you can't use it right away.
3: Usefulness. This was especially important when I was starting out. Some modules, say a sequencer or logic, are useless by themselves, but others have at least limited functions by themselves--you can play a little bit with an oscillator or a resonant filter, for example. Some modules like complex oscillators with built-in modulation and "voices" can do a lot on their own, if your planned rack includes any of those you might want to prioritize them when you start. This changes as your rack grows though. A sequencer with no oscillators is useless, but once you have one or more oscillators now a sequencer becomes useful. I think uselfuless also applies to any weak points in your system. When you're just starting out everything is a weak point. But before too long you will idenitify specific needs. Right now my biggest need is envelopes, so that's what I'm planning to buy next. I often ask myself the question: "I've got $X to spend. What module(s) can I get for that which will add the most functionality to my rack?".
4: Balance of utility & fun/inspiration. Some modules are fun and exciting to play with: a fancy new filter, a complicated oscillator, granular processor, DSP, etc. Others are boring but are 100% necessary utilities, like I/Os, mults, VCAs, attenuverters, logic, offsets, S&H, clock tools, etc. I feel it's important to strike a balance between them. Utilities are super important but a rack full of nothing but them is boring and uninspiring. And likewise you might have a bunch of really cool oscillators and FX but without the nuts and bolts that hold everything together you can't do very much with it. So I try and maintain a balance of purchasing interesting and inspiring modules while also keeping up with the utility stuff at the same time.
I'd also suggest this starting out: make sure that at least some of your patch cables are stackable or pick up some splitters. This avoids the need for passive mults in your rack, saving you both space and money.
