How neccessary is a car in Singapore

Family of four.

Looking at leasing SGD2k per month

or is public transport/taxis good enough that hiring a car once or twice a month is a btter option whenever you need it

do condos come with parking spaces or is that even more on top

Out of all cities in this world, Singapore is certainly the place with least need (and highest cost) for a car.
Public transport is excellent and taxis are cheap. You won't even need a rented car (except for trips to Malaysia - if you really want to drive there yourself)!

most condos provide at least 1 complimentary parking space but you should check this with the housing agent or landlord.  If your workplace, home and/or school is near to a MRT station, you probably will not need a car.  Having a car will give you more convenience and the freedom to move around but it is a more costly option then taking public transport.

For most people, not necessary. The MRT system is great but there is also the bus system which only takes a tiny bit longer to work out - and both use the same ez-link ticketing system (stored value card, tap in/tap out).

HOWEVER you may consider one if your kids do a lot of extra-curricular activities, especially if you're not planning to employ a helper.

Shouldn't be too hard to arrange a parking space at home. Parking at your destination is another matter - that can really add up although apparently those with cars know tricks to keep it under control. Hope you're good at reverse parking in tight spaces ;)