The first lens is for 35mm film, second lens is for 6x6 format 120/220 film.
To replace the Steinheil Cassar, any 75 or 80mm lens will work (Schneider, Rodenstock, Nikon, etc). Hopefully the Steinheil has a standard 39mm Leica mounting thread. Otherwise you will also have to buy a replacement lens panel for to match the mounting thread of the new lens.

If it is the same lens as this one, you will need to buy a new lens board for your new lens. 30mm mounting thread is NOT a standard size.
http://www.ebay.com/itm/STEINHEIL-75...item53f45c72e1

FYI the 2 common lens thread mounts sizes you will find in lenses will be 25mm Schneider and 39mm Leica thread.

ps the M600 is a nice enlarger, I still have mine, 40 years old and still working fine.