Phonenut:
If you're using real CO lines (i.e. copper from local phone company) and want your KSU to work for normal, everyday use, a 400D line card should work. They are cheap. For more modern uses (i.e. dial tone from your cable company, Vonage), I would suggest a Western Electric 400G or 400H which are also inexpensive. A 400H will even work with a MagicJack I discovered. You could also use the a San/Bar 4000F or higher, which I believe is roughly equivalent to the WE 400G-H series line cards.
Here's a link that lists some of the 400+ line cards and their uses.
Western Electric 1A2 key telephone reference.
