Question about Samsung 860 EVO Basic

Yes, your iMac must have already had an HDD 2.5" installed, then no problem, provided you have the glue for the glass. If you have a flash memory installed, then you have to organise the cable first, which is not impossible with original Apple parts, but not exactly easy. Shouldn't be a problem otherwise, if you pay attention to the few small things, Google ifixit, they'll show you how it's done, otherwise there's probably a few companies that will also do a conversion, if Digitec offers something like that super, otherwise you can take a look in Güterstrasse in Basel.