I am looking for a lower unit on a merc 7.5

I have fixed many things in my life and turned a lot of wrenches, but I have never worked on a boat motor. I bought a boat this weekend and it came with a Mercury 7.5 The previous owner said that the "lower unit is trashed". I would appreciate any advice on how I should fix this problem. Where do I start?

Thanks,
Kevin
West of Little Rock