If a sim dies can I download a new version of them to my neighborhood?

You can, yes, but you can also click on their headstone and select "keep soul tethered to this world" (or something like that) and it will keep them around indefinitely. The game will notify you when theyre about to "move on" so you can do it again.

you can also use a spellcaster to bring them back to life with the dedeathify spell.