Personally I'd choose the mantis over the rackbrute for a few reasons - I don't like it aesthetically and it uses a power input module that reduces the amount of hp available immediately, the mantis is also efffectively 33hp bigger (power is 5hp)
However, as you are a minibrute owner, I can also see the appeal of it... and it can be connected to the minibrute and there's a bag
that fits both together and it's a bit cheaper
As for modules - to start with I would concentrate on modulation and utility as that will add most to the minibrute
So I'd either go for maths (or Pams) and a quad cascading vca (the intellijel that you mentioned, but also look at veils)
I'd also get mutable links and kinks as they are such a great utility started package
Then and only then would I consider adding an extra voice - plaits is a pretty decent choice - especially as you could use the model selection cv input to sequence the model - so you can hit it quite fast and change the percussion instrument between hits
Then I think I'd be tempted to grab an analog filter, probably a multi-mode one, and whichever of Pams or Maths I didn't get earlier
And a disting!!! - it's just so handy to have, really, really useful
And then some basic utilities - sequential switch, clock divider, maybe a matrix mixer
But go slowly and have fun
"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