@HGSynth has done a great job there with advice
I'd also take a quick look at my signature - & spend some time thinking about it... it's a quick guide to getting the most versatility for the least expense... & it scales well too...
bigger case - check... can't say I'm a fan of the rackbrutes, as a starter case I think the mantis wins every time on every level (hp/cost/manufacturer reputation/decent power supply/no hideous rack wart), but as an addition to a minibrute they make a lot of sense, but only if you're going to connect them physically... if you're not going to connect them and have the rack to the side of the minibrute then the mantis wins (& they come in black now too)
definitely worth investigating more modulation sources and utilities...
Maths is brilliant - it's got decent ergonomics and a decent feature set - which in itself is invaluable, but most importantly there's a lot of educational material - videos and especially the 'maths illustrated supplement' which is an excellent primer for patch programming and patching in general - especially when time is spent trying to understand what, why and how it's doing what it's doing...
the addition of another modulation source such as batumi or zadar is a great idea!
I'd probably go for the Happy Nerding 3 x MIA over the TUL as you get an extra channel in the same hp and ergonomics is still good
a quad cascading vca is a investment - I'd go for a veils clone myself - you can never have too many vcas!!!! yjru're really useful for modulation as well as audio! and they can be used to attenuate modular level down a bit before hitting whatever you're currently using to mix the minibrute and the drum machine
if you can find one, a kinks clone is always useful
if you're thinking of using the turing machine as a pitch sequencer at all, you'll almost definitely want to add a quantizer
I always like a matrix mixer (or 2) - great for combining modulation, setting up feedback loops into sound modifiers, send/return of effects etc etc
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities