4 GB of RAM seems to be the bare minimum needed to play the game fluently. If you happen to only have 2 GB, most laptops are upgradable and RAM doesn't cost very much.
In order to have room for all of the expansion packs, you should probably have at least 20 GB of hard drive space. (Leave some room for new ones). This shouldn't be a problem if your laptop is going to be a new one. Probably more like 30-40GB once every single stuff pack/expansion pack is out. Once again, shouldn't be a problem. And if it is, you can always upgrade your hard drive to a 500 GB one, which doesn't cost very much.
The video card, however, can be the deal breaker. You cannot upgrade the video card on your laptop. Most laptops come with an integrated graphics card. If this card is the Intel HD (most common graphics card), then you can play the Sims on low-medium settings with advanced rendering disabled. Ideally, you'd shoot for a middle-class, dedicated laptop graphics card so you can play the Sims 3 on medium-high settings (depending on how many expansions). To check if your graphics card is adequate, you can go to Google and type in, "________ graphics card review" and one of the first sites on the list should be Notebookcheck.org (or .net?) and should provide the necessary information.
It would help to know the specs of the laptop you're getting. Perhaps a link to it (if that's allowed)?
I remember when I bought the laptop I'm using now, I had to do quite a bit of research to make sure I can play the games I wanted to (including games that are more demanding than Sims 3).