A few things...first up, the mixing really needs to be internal. Buchla's mixers also did quite a bit with CV over spatialization and weren't merely level controls. Have a look at https://www.modulargrid.net/u/buchla-system-interface-model-227 , which is the original 200/300 mixer I'm familiar with. Note that you have control not merely over stereo panning, but quadrophonic spatialization, in addition to a bunch of other functions that, to be honest, really bring a level of convenience to a synth mixer that you mostly don't see now. The closest thing to this would probably be Koma's quad performance mixer, which would make a decent stand-in for something like the 227.
Filters...if we're not talking LPGs but actual, typical VCFs, you can probably do with less. Buchla's gear was more about wave combination/shaping, so typical subtractive methods didn't happen much. But note that: 'much'. There were still a few filters that made it into typical Buchla architecture that we'd recognize, and of course there's all of the spectral-type filtering that was a lot more common starting in the 200 series. The Bark filters definitely satisfy that need. Also, there's only one thing I can think of that's like a critically-significant Buchla 'filter', the 296 (see https://www.modulargrid.net/u/buchla-296), and that would be the Frap Fumana. Not cheap, but definitely in a similar vein to the 296 in function and usage methods.
Lastly, the controller. If you're not going to use touchplates, you're not going to get the same...ah, hard to describe...'behavior' out of an iPad. The iPad (or any other tablet) will correct a lot of the weirdnesses that the various touchplate controllers brought to playing the Buchla. For example, one thing I was warned about is that setting certain touchplate voltages before spending some time settling into the studio will likely later result in values that aren't what you had in mind. Yes, these things could be that biologically sensitive! They literally bring you into the instrument circuits themselves, if that makes sense.
Also lastly (yeah, yeah...), it could be smaller. Yes, big 200/300 systems were hulking things, but Don built those modules in a larger format (4U-ish), and you'll notice that present-day 200e setups are pretty small, physically. Even the System 200e we see Marton Subotnick using in "IDOW" is a smallish affair when compared to his original 100 rig. You can do more with less in a typical West Coast setup, to be honest, so it shouldn't be too necessary to go quite so big here.
Mind you, Buchla stuff is getting cheaper if you go outside of Buchla itself. Northern Light, Roman Filippov, et al are gradually getting the Buchla-format pricing under a modicum of control, so one could make do with a few of the nosebleed-pricey Buchla modules augmented by a lot of other not-Buchla that doesn't cost so much.
