1992 115hp Mariner Water in Lower Unit - Reverse is GONE

Well I took my boat out of the water last week so we could take it on vacation with us. As I was pulling out of the dock, the reverse grinded then now won't go into reverse at all. It will only grind, so I pushed the boat back and got it where I wanted. I brought the boat home and decided to check lower unit hear oil......brown(was green going in this spring) and I found metal shavings. Forward works perfect(thank god), so I am guessing I have water coming in from somewhere. What is the best place to start looking? Also I am mechanically inclined....but I am by no means a mechanic....replacing the lower unit gear(reverse) something the semi mechanically inclined person can do with a manual? Yes I do have one, and I do 90% of all my work on it. If I can't do this roughly does anyone have an idea of what I could be looking at on price? I know this is a vague question, but I want a rough idea 300 400? Lets just say I wanted the reverse gear installed and a leak test done....would I be looking at something under a grand? Any help would be appreciated to help me start looking into this project, I want it done before the season is over so I need to start yesterday.