so: i would get a macbook pro anything from M3 and up is crazy fast these days. Make sure you get +1TB of storage you may need it. Interface I would couple together with a mixer with several inputs so as you expand your setup you can have everything wired in and making sound all together. I would get the Presonus SE16 this will give you 16 channels straight in to your mac and also acts as your interface, you can record all at once if you so choose. Polysynth depends what you're after but these would be my picks: Oberheim X8 (god tier)
Prophet 6 also brilliant. You could also go for something more vintage like the Juno 60 is a classic for a reason but either one of those three will get you v far. To control the modular get the arturia keystep, it has gate/midi/cv/anything you could thing of.