You'll need booster pump only if your tap pressure is not enough. 3-4 stage is enough.
Mecanical + carbon + RO
unit. You can add the fourth stage DI
Or get as you said a kind of Mech+chem+chem+ro
In my case it's a kind of old RODI
unit 6 stages
I plan to remove the antitase thing, I don't want to pay for this, so I'll get 5 stages:
The chem part is important, as free chlorine kills the thin ro