The most old-school: Casio SK-1 But no MIDI, not programmable, no anything esle than Old-Schoolness
In a different category, have you checked the Elektron MachineDrum? If you don't want the further options, some of the first versions can be found for not so expensive -nothing in common with the Alain's Boss though
The MD is quite flexible in terms of sound. You can make it sound very different ways. It's one of these beasts you can dig and find out tricks from for years. The sequencer is reaal cool too.
I would say its mains drawbacks are:
- 16th step resolution only
- no velocity onboard, but available through MIDI IIRC
But maybe it's an overshoot... plus it's not "old schol out of the box"...