If you're only going to have one module to start out with, I would go
for a filter. You can use that to process all your other gear, even
without the A190... just make sure you have some other way of
controlling the level of signal your sending to it. Filters are
pretty essential, and there are a lot of different ones to choose
from. If you get one that self-oscillates, you can also use it as a
simple sound source.

At the most basic level, you're going to need an oscillator, an lfo,
an envelope, a vca, and a filter just to have a single complete synth
voice (with no direct way of controlling it). Since your budget is
rather restricted, perhaps you should consider sticking with sound
processing modules for now, and later expand with vco's and modulation
sources. Look at filters, waveshaper, frequency shifter, ring
modulator, waveform processor, wave multiplier, BBD, etc.