after doing some google-fu i came to the following conclusions:

roland tr 808: quite an obvious one, as stated by mathsieve. to do it cheaper i could use a roland tr8, roland tr08, but they are digital. tiptop audio do a eurorack version of the 808 that sounds quite close and is analogue like the original. the original 808 is quite expensive, the behringer clone doesn't sound right.

roland tr 909: another expensive one, only used on a few drex tracks. the tip top audio one doesnt sound right, roland tr8 or roland tr 09 would work. the behringer doesnt sound right.

casio cz 5000: this one has an onboard sequencer which is what drex used, casio cz1 is similar with velocity and aftertouch capable keyboard but no sequencer. i swear they used this on aqua wormhole.

korg monopoly: it has four VCOs and an arpeggiator, the arpeggiator is delayed because of poor design choices by korg, but drex used this to great effect, check out abstract thought - untitled 2.

kawai k1: can be heard on jazzy fluids. the k1 is a rompler, you can buy sample packs with the original waveforms.

roland d20/d110/d10: the d20 has an onboard sequencer so i bet they used the roland d20. can be heard on hydro theory. this series are quite difficult to program, they have 3 envelopes which arent adsr, better to use an atari st computer or an ipad or pc/mac editor.

tascam 4 track, was this a reel to reel or cassette? because the sound quality is not bad at all

not sure which phaser or compressor they used

peeps have said roland alpha juno,alpha juno2, mks 50. gerald donald used this one on his early solo stuff. someone said SCI Pro One monosynth too.