If you can provide additional light like Steve suggests, and you say you can, it should/could be OK. However, I would possibly not start out with PP and growing Lithops from seed. Most PP tend to grow into plants that are not really indoor suited, though there are at least a few that will take the temperatures you get in winter so you might even be able to grow the right ones outside - the wetness/humidity might be the problem rather than the cold. I would definitely not be the first type of cactus that would come to mind for growing indoors, there are many other types of cacti (Mammillaria, Gymnocalycium, Parodia, etc.) that would seem better suited to grow inside on a windowsill.
Lithops require quite specific growing conditions which change with species. I'd say it is worth a try, but maybe first with a live plant before you start growing from seed.
