It's hard to give advice as to what to get as an essential module. That's going to depend mostly on what you want to do with your set-up.
But let's look at the Minibrute. It has a sequencer, envelopes, filters, oscillators. However, it's lacking in effects. Effects aren't as sexy as an uber-VCO. But they are the most underrated part of synth sound design. The Koma Field Kit FX would be the first thing I look at. I don't own one. But if I had a MiniBrute, it's looks like a great pairing for under 300Euros.
It's a standalone box and won't eat into your Eurorack case. Plus you'll get use out of it beyond your basic set-up. You can still add other effects like a Clouds, dedicated reverb, high-end delays, etc. to your actual case and they will all integrate together.
The next modules I would purchase would be some multi-function modules. Think of them as a tasting flight for dedicated modules. The Ornaments & Crime and Temps Utile are my two favorites. Between the two, you get a TON of useful features for additional sequencing, envelopes, quantizing, Euclidean patterns, clock multiplying/dividing, LFOs, etc. You can also add an Expert Sleepers Disting for under 200Euros. The Disting has a lot of features but can be maddening to work with the interface.
The O&C, T_U, and Disting sum up to 18 Eurorack spaces... so they are going to fit into almost any starter system.
Maths or the Befaco Rampage are great. At a minimum they bother offer two slew limiters. But they both can do so much more. However either module takes up quite a bit of HP for their functionality.
I own Ornaments & Crime, Temps Utile, Maths, a clone of Clouds (the Monsoon), and the Disting just for reference.