I you want to learn how synths work, I suggest you buy a midi controller and assign each knob on your softsynth to a knob on your midi controller. That would be a lot cheaper than buying a hardware synth The BCR2000 has a lot of knobs and is quite cheap too.
Cheap second hand synths are: Roland Juno 106, Novation KS rack, Novation Nova, Nord Lead rack (already a little bit more expensive) Korg Poly 61(m), Korg Poly 800 etc. These are the only ones I can think of at the moment.
If you really want to buy a hardware synth to learn basic (subtractive) synthesis, you should go for the Nova or KS rack. They'll fit your needs the most I think. The Nova even has FM possibilities, but they're a bit limited.
But remember: once you go hardware, you'll never want to go back to software
edit: if you REALLY want to learn synthesis, look for a Nord Modular You can find them for €500-700.