Yeah, the MScale is definitely a must for controlling the M32 with the Eurorack and vice-versa. Next, I think beefing up the modulation capabilities is key here. Doepfer's A-143-1 gives a lot of options for that, plus it comes in very cheaply at 169EUR, which takes us to 229EUR.
Then, I think something that allows you to beef up the M32's sound is going to be critical, namely another VCO. Sticking with Doepfer (best value for function if you're in Europe, I think), the A-110-1 at 117EUR is a decent choice. You don't need anything super-fancy, really, just some different waveforms, sync, and CV mods. So, 346EUR, within your budget frame.
And yeah, Doepfer stuff is just fine. It's rather basic, but when you need basic, it more than fills the bill. Dieter did these things right...so right, that they're what kicked all of this modular craziness back into gear some 20+ years ago!