@NumberedBeats If this laptop has been maintained well, it should be right on the border of what can run Sims 4, all packs included, on ultra graphics settings. The caveat is that we haven't really seen the results of today's patch, which should improve performance overall but may have side effects here and there. Stardew Valley should be fine too. This laptop's graphics card is a bit slower than the recommendation for Dreamlight Valley, but it's well above the minimum, so you should be able to play fine, just not necessarily on ultra graphics settings.
Sims 4 may well lag here and there on this laptop (again, depending on how much of an improvement we see from today's patch), but it shouldn't be the hardware's fault, again if the computer has been properly maintained. The game lags at least a little on any system.
If the seller is local to you, I would strongly recommend running a benchmark or two on the laptop before buying it. Specifically, Steam offers a free demo of 3DMark, and Time Spy is a good test of the hardware. If it crashes while running, or the score is much lower than predicted (a little lower is fine), then don't buy the laptop, or at least investigate further.
And yes, you could remove the HDD and slot in your SSD IF it's a 2.5" drive. I would guess this laptop doesn't have a free M.2 slot given its age, but the HDD bay is almost certainly 2.5", and the bay and cable are standard and compatible with any 2.5" drive that uses the SATA interface.
If you're going to upgrade, I'd suggest adding more RAM as well. Another 8 GB module could be as cheap as $20, and since you're already opening up the chassis, it's easy to install both at the same time. Double-check the configuration though—it could be 1x8 GB or 2x4 GB, and in the latter case, you might want a 2x8 GB kit. (12 GB is still better than 8 though.) The Task Manager should report the configuration.
