Hello,
You need :
Power supplie => uZeus
Drum sequencer (3 voices) => Grids
A Mixer => Tiptop Mixz.
Search for this ones in priority, so you can have a fully functionnal mini drum system.
After this, search for some basic synth parts : VCO/VCA/ENV...
Try to stick to cheap modules to begin with (Doepfer is really a good deal...).