The Tiptop uZEUS is a good starter. I still have mine. Do monitor your modules' mA's though and set the jumper accordingly. RTFM. (Funny things will happen once you are coming close to the limit of your PSU. Modules will refuse to start intermittently and are seemingly dead. Others appear stuck or start acting funny after a couple of minutes... That's why it's a good idea to plan for 20-30% more power than your modules add up to. Some have a hefty inrush current.)
Modules I use all the time are multiples (A-180-2 and Mult), the Ladik M-120 Attenuverting Mixer and Uoki Toki 4LFO. All of these pack a lot of needed functionality in a small space which is important in a 6Ux84 case. I used to have 4 mults all patched up and still wanted more. This was much alleviated by a newly aquired A-138m Matrix Mixer. A Disting I don't have but I can see one in my future.
A note on Maths. It's a great module and gets best marks around here. Still I would recommend getting Function. It's a looping envelope with VC-able attack and decay/a slew limiter/LFO/oscillator/S&H/envelope follower /trigger source/whatever, all in 8HP. Which basically means you could build a synth with just a handful of these. Allow a few month to get bored by all of that , then blow twice as much on Maths. Or get a second Function. And, your rack space is precious. Just my 0,02 EUR of course.