Sims 3 Pc system

@pricthyre  My personal experience isn't necessarily helpful here: I played on an old Mac for a long time, and now I have a Bootcamped MacBook Pro and a gaming PC that's way over the top for any Sims game.  TS3 runs great on both.

If you want to run all packs on high/ultra settings, don't go any lower than an Nvidia 1050 or 1650, or an AMD RX 560.  There's not much benefit to getting a faster card for the game itself—a 1050 ti will handle all packs together on ultra settings.  However, if you like high poly (high-resolution) custom content, you might benefit from an Nviida 1660 (or 1060 if you can find one), or an AMD 570 or 580.  More importantly, the cards in the next tier down from the 1050 and 560 do usually struggle with Pets and Seasons, even on lower settings.

For the rest of the specs, 8 GB RAM is enough, as is any newish Intel Core (as in, i-something) or AMD Ryzen that doesn't have a U or Y in its model number.  While Sims 3 doesn't technically need to be installed on a solid state drive, it will load significantly faster on one, so everything from the initial load time to opening CAS or Build/Buy to messing around in Edit Town will be faster.  An SSD of at least 256 GB should be okay if you're good at managing space, and 512 GB would be more than you'd ever use for the game.

In general, laptops that will run TS3 on ultra settings can usually be found for $700 U.S. or lower, and under $600 if there's a convenient sale.  Desktops will be a bit cheaper but not include a monitor.  Prices are higher in other countries; how much higher depends on the country.

When you're ready to buy, feel free to come back and ask for more advice, link a couple of models you're considering, or just ask for some options so you can see what's out there.  If you list your budget and country, as well as any other details that matter to you, I can have a look around and list a few good laptops or desktops.