A few random thoughts:
--Markyboard is right. There are oscillators in the Eurorack format that aren't...oscillators. Well, they are, but they aren't. I've got a Mutable Instruments Elements that I would be hard pressed to categorize. Honestly, it straddles the line between beeps & bloops (which I have little patience for) and music, in the sense that it can be tuned to play actual notes. Face it, you've got about 999 manufacturers out there trying to differentiate themselves from one another and so they go further and further afield trying to come up with something the other guys don't have. Some of it sounds like crap (to my ears) and some of it works (again, to my ears). Your best bet is to listen to a bunch of YouTube demos, because I swear some of the people who do the demos go out of their way to make the most horrible, screechy noises they can...so if the first video doesn't sound like what you're looking for, try another. If, after three or four demos, maybe five if you're really trying hard to like a piece, if after all that it still sounds like crap to you, move on. These things are so bloody versatile that it's possible to get both good and bad noises out of them. Or maybe it really was a bad design to begin with. (I note that one of the modules I least liked the sound of--no matter how many demos I listened to--has been discontinued. Maybe I wasn't the only one who didn't like it.)
--Certain manufacturers don't help themselves to be taken seriously by describing their products with word salad like this: "Just Friends discussing the many facets of their empathic geometry. In generating manifold envelopes, projecting impulses, cycling on parallel gradients. Together navigate the sixth nexus into your own personal patch communion." Uh...say wha'? That's a direct cut and paste from the Mannequins Just Friends page. So tell me, just exactly what does a Just Friends do, anyway? Hell if I know. I'm not even sure they know. Back to YouTube.
--Some companies have panel nomenclature that is all but unreadable. I don't know if I'll ever buy something from Make Noise, for instance, because I can't read their damned labels. Edgy? Attitude? I care not for these things. I care about coaxing music out of hardware and if I can't read the stupid labels because some nitwit decided to get creative with the font, then that piece is a non-starter for me.
--Eurorack is like kudzu. It grows and grows and nothing stops it. By more space than you think you'll need. Trust me on this one.
--I thought, as you do, that I'd use the Behringer Model D for oscillators and such. The reality is that I can't get into the circuit where I want to--the patch points simply aren't there. I'm handy with a soldering iron and have considered making a breakout box so as to get into more places in the D's circuitry, but...nah. I just treat it as its own thing and use it in parallel--not in series with--the other Eurothings.
--I'm here to certify that it is possible to make actual music, as in real notes that have pitches, with Euro hardware. I use MIDI to drive the Eurorack, just like I do the Model D. Works great.
--A sequencer that you can actually touch, as opposed to fiddle with awkwardly after diving through ten layers of menus, is a good thing. But...sequencers can get big. Ooops...there goes another 30-40 HP!
--People tell you that, "You can never make the same sound twice on Eurorack." Uh...bullshit. It's no worse than a Moog modular. Or a Minimoog, for that matter. Yes, you have to twist knobs to get there, but if Keith Emerson and Rick Wakeman could do it, then we can do it. Note also that there are pieces on the market that have patch memory, so there's progress being made on the convenience/speed front. If there's something that I think might be tricky to repeat, I whip out my phone and take a picture of the settings I used. Easy.
--The complaints about tight knob spacings are true...sometimes. Some modules are okay to live with, while others have you bumping knobs B and C every time you reach for A. It varies.
--I use 84HP/19" rack mount adapters because everything else I have is 19" and I'll be damned if I'll start another rack format if I don't have to.
--Having just about every potential patch point that there is allows versatility beyond your wildest dreams. As much as I love my Moogs, they (and the Behringer) don't let you do everything that can be done.
--Eurorack gets expensive. Eurorack gets big. Eurorack is more fun than the law allows...and can be much more than mere beeps and bloops. Just think of it as the Mini-Me version of Keith Emerson's wall-o'-Moog.