I think you should check into this deal carefully with the plan to run at the first opportunity.
The first thing to check out is the condition of the trailer. While boat trailers are expensive to buy ($3000+) this one may also be rotted as trailers tend to deteriorate much more quickly than do fiberglass boats.
The next thing is to ask yourself why you want a trailer and if you really need it. Keelboats are typically stored at boat yard in the off season and there is little or no need for a trailer. Steering clear of a rotting boat and trailer with the associated disposal headaches may be the safest bet. You will probably also find that having a trailer does not save you launching fees as most keel boat trailers are not suitable for ramp launching of a keelboat.
With all this if the Mirage is still a boat you want then you might consider buying it.
The C&C24 is a bigger and heavier boat than the Mirage so much of the gear would be usable on the Mirage if it is still in usable condition. Blocks, winches, spinnaker pole, etc....
If you are hoping to use the spinnaker and pole from the C&C24 you should know that the pole is 1.5 feet longer than the one for a Mirage 24. It can be cut down to size pretty easily so you might be OK. Also the spinnaker would be 1 foot longer in hoist and nearly 3 feet wider than a Mirage 24 spinnaker so may need a recut.
There are a host of questions but it seems to me that the burden of having a junk boat would make the deal not worthwhile. If the deal is still worthwhile on the merits of the Mirage 24 by itself then maybe you should consider that.
If yor are stuck on the idea of a trailer then maybe look for a MacGregor 25. There are lots of these sitting in driveways everywhere and they are cheap.