how early can I put food into a slow-cooker?
Yes.  A typical slow cooker's low setting ranges between 8-10 hours, which is within the range of your listed times.

All (or nearly all) slow cookers come with a removable ceramic bowl/insert that the food cooks in.  Most families do most of the preparation in the evening, putting everything into that bowl, and putting the bowl into the refrigerator.  Then when they wake up in the morning, they put the bowl back into the slow cooker and start it up.

Your bigger concern should be what size of slow cooker to get.  If you don't fill up that ceramic bowl to at least 1/2-2/3's full, you can end up burning the entire dinner.