My 2 Cents... Having been in modular synthesis now for 3 or so years... and making mistakes, etc. Here is where I would suggest to start. Now, before I begin, I will say that I am really pushing Make Noise products, but that's because you can use their products to learn both East Coast and West Coast synthesis techniques. There is lots of great videos covering their products to help you learn some great synthesis theory. So, if i were to do it all over again.... 0 Coast, a Wogglebug, Maths, and maybe some 2 hp reverb/delay modules. Gives you a good start! A great start actually. And you can use all of these modules within your Eurorack system as it grows. I would suggest building your own case with a intelligel power supply if you want to save on a case. Do research, lots of it, before purchasing anything. Buy modules that are flexible and give you lots of bang/options for your dollars.
So why Wogglebug and Maths... they are great cv generators that are extremely flexible in how they can be applied and used within a modular system. Wogglebug can generate random events, and clocking, and cv. It also has oscillators of the experimental nature as well. Maths is Maths...;. and most likely, you will end up with one anyway. 0Coast is great as it has oscillator, stepped cv, clocking, a Maths inspired cv generator, and an AD envelope generator. It has lots of options as to what can be cv controlled by either within its own system or from other eurorack modules.
If you are looking for some modules that are relatively affordable, look at 2hp for your envelope generators, Amp modules, filters, sequencers... etc. Noise engineering also makes some good sequencers that are both flexible and relatively affordable. Happy Music Making.