I am taking the first nervous steps into Modular.

cool + welcome!

My plan is now to get a Rackbrute 3U as my next steps and am thinking about what modules to pair with the minibrute.

No - see below

My plan is to spend around £300 per month to get one or two new modules every month give me time to get the hang of what they do and how to use them as well as still pay the rent!

sounds like a sensible plan!

Having done some research I have filled the 3U with the modules in the order that I plan to buy them (this is not the order that I plan to place them in the end)
I know that I could obviously get more in a 6U rack but think that there is a creative plus (and financial one too) to limiting what you have in some ways and this is only one part of my music making set up.
What do you guys think?
Would you suggest a different oscillator or filter to contrast the Brute 2?

will wait and comment when you've posted the link

re case: look at the total cost of ownership - price/hp - the 3u is way more expensive than the 6u and you will run out of space in the 3u very very quickly - personally I would get a mantis - way better value for money than the rackbrute - and no rack wart - but each to their own! - I'd recommend getting a jar (or dedicated savings account) and putting the rack rent* for each module you buy in it - by the time the current case is full the next case is paid for!

*rack rent = hp of module * (cost of case/(hp of case - hp of rack wart))

Do I have too much modulation and not enough sound source or vice versa?

almost definitely not enough utility modules

"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