Hi, I know of two schools on the western side of the lake that do 18 months to 5 years, although one does not have primary (they stop at age 6, after which the kids need to move on to another primary school).
Montessori House of Kids, at Grüt Park (former site of Grüt Farm, between Adliswil & Wollishofen) is the one without the primary school.
Rietberg Montessori School has, according to their website, ages 3 - 12 but I have heard that they recently opened a toddler class (from 18 months). They're right near Rietberg Museum, on the #7 tram line (just two or three stops from Bahnhof Enge).
There is a third bilingual Montessori school I know, over on the eastern side of the lake (near Kreuzplatz & Hegibachplatz),
d'Insle Montessori School that also does toddlers through to the end of primary school.
I have three kids; the older two go to d'Insle, the youngest to House of Kids. The main reason for the split is that there was just no space at d'Insle for the youngest when we wanted him to start. Another component of our decision, though, was that if all three were at d'Insle I'd have to bring all three to school and back each day ...

This way, my wife takes the youngest & I'm left with just the two older children. For a few months I had to take all three, dropping the youngest at a Swiss krippe before getting the other two to school, and it was a nightmare! Of course, coming from the other side of the lake, it was half an hour with all three on various modes of public transport ...