Well, the price was pretty good.. you could probably sell the trailer for about that much and come out even.
For the boat.. man.. there's a lot to deal with. First, you are going to have to dry it out, and that's likely to take months. Some might suggest drilling a number of 1/8"-ish holes through the laminate to speed up the process. I might agree with them, given the amount of fiberglassing you are going to be doing anyway.
Step next is gong to be trying to find out how much water damage has been done. Clearly, the entire bottom is delaminated, as evidenced by the 2" 'sag' you described.
OK, this part is only half tongue-in-cheek. First, get a big sawsall. Maybe a nice chainsaw, even. Insert the saw at about the waterline, and cut horizontally around the boat, following the waterline until you return to the original spot. Remove the bottom half. Then build in a new one.
Of course, that's silly. But you'll be doing most of that same job, only in pieces. You'll grind the fiberglass away from the keel joint.. and not just the front of it. If the whole keel is dropping, then the whole keel is suspect. Then you'll have to replace any rotted coring you find, which will likely be significant.
To replace rotted core, here's the short version: cut/grind/peel the outer laminate layer back until you get to good core. grind out bad core. Replace with good core. That third step is a real pain in areas with compound bends (such as, for instance, keel joints and waterlines). Then fiberglass over the new core. Then barrier coat, then bottom paint. Some would suggest the use of gelcoat in there, between the fiberglass and barrier coat, and there are intelligent arguments both ways on that issue.
Now, there's the keel. Obviously you'll have to figure out where the water got. Most likely, the entire thing is delaminated. Now, maybe you can use some GitRot and then follow with a vaccum-bag resin treatment. But likely, there is a buildup of crud in there that will compromise the resin bond. Which means stripping off the existing glass, cleaning, and reglassing.
Or here's a third option... and probably the one I'd go with:
Just fix that big hole (grind, scarf, reglass). Tighten the keel bolts, maybe reinforce their backing plates on the boat. Put some barrier coat on the glass, and sail until the keel falls off. Sail on 'friendly' days, keep your jacket on, and know that if the keel falls off you'll need to drop sail in a hurry. I know a guy who sailed a long way without a keel. It's not wise, maybe, but it can be done. And it's better than both a) not having a boat while you fix your mac and b) spending three months and 8 grand fixing a $3000 dollar boat.
Oh, and when everything is said and done, sell that trailer for $300 so you can smile about not getting taken on the price