best gaming desktop for sims 3 and 4

@booklover240  PCIe SSDs are much faster than M.2 SATA SSDs, and since you're already used to the PCIe in your macbook air, I understand why you'd want to stick with that.  PCIe models are also more expensive than the alternatives, of course, but if the speed makes a difference to you, then it's probably worth it.  (If you'd like some idea of how your current SSD compares to the models available for your new computer, click About This Mac > System Report > Storage, and list the device name of the drive.  I couldn't find it myself in ten minutes of googling.)

If you want to run TS3 off an SSD, which would be much faster than using an HDD, you'll want one that's at least 512 GB.  Between the program files and the game folder that you'd like to fill with custom content (as well as saves and everything else), you'll run out of space quickly on a 256 GB drive.  And you can certainly install TS4 on an HDD without seeing any drop in performance, but its game folder will land in Documents as well, which you'll want on the SSD for when you're playing TS3.

If this computer is only for Sims, you don't need a second drive besides the larger SSD.  But you might prefer to have the HDD as convenient internal storage for other data.  It's not a big deal to get an external storage device later, either.  For reference, I bought a 4 TB external for $110, which is the same as it costs to add a 4 TB 7200 RPM HDD on the site I was browsing.  (You can get a 1 TB drive for $37 or two of them for $52, or you can pay a bit more for a higher quality drive.)

The other point to consider is that these computers can be upgraded later, if and when you decide to do so.