I can only give the US perspective (don't know what's available in Singapore), but it is an annoying issue. I am a complete freak about this, so sadly I know a good deal about it.
You are right that the baby seats only last for the first bunch of months (it is a height issue, actually not a weight issue -- once the top of the head is less than an inch from the plastic edge of the car seat, you need to trade up. This happened between 4 and 6 months for both of my kids). I think Graco now makes a bigger baby infant seat (safe seat? something like that) that is bigger -- more awkward to schelp around, but I believe it is still supposed to snap into strollers. We bought a sit and stroll, which is a combination carseat / stroller (the wheels tuck up under the seat when it goes into the car). It is neither a very good stroller nor a very convenient car seat, but it is the best solution I know of -- once you get where you're going, you can roll the kid around, but you still have a car seat for the return trip. It can be used back-facing for infants and toddlers, as well as front-facing for bigger toddlers and preschoolers, and my son used it up to around 4 (when his little sister graduated in to it). You'll get many years of use from it.
For encyclopedic coverage of the car seat / car seat in taxi issue, there is an amazing website (rideonthesafeside.com), which covers the various options incredibly thoroughly.
Congratualtions and good luck!