Generally you want to spread out your buys, and start from a corner with the essentials. This way you can alter your plans as you learn more about your gear.

What you start with depends on what you already have, and the bare minimum to get something going. That probably means you need to start with power and one or more voice.

Do you have a sequencer available? If not, the Pam's probably a must. (You probably want the Pam anyway though, it's very useful.)

What other sources of sounds do you have? Can it be combined with eurorack? (Some synths provide CV points.) Maybe you could start with one Drum2 and either the A-111-6 or the Beehive, depending on which you like best, but it really depends on what you already have.

Do you have a mixer, and does it handle the level of audio coming from eurorack (it's high compared to most other synths)?

You probably want an effect module too, and the FX Aid should go a long way there. It's not essential, but gives you lot more options.

Hope this gives you some ideas ^_^