IMHO, in this order (bearing in mind you have a few months to deal with all this)
1) Fix trailer, you should be able to do this but might need a little help, poss take some pics of various parts you are concerned about and post them here? The problem, if you don't feel you can do it yourself, is that it's stuck where it is! You could maybe find someone at the marina? Or get a friendly mobile mechanic?
2) Don't bother changing battery untill you have tried charging it up and seing if it holds the charge.
3) Personally i would check all the electrics and just change them if they looked corroded or did not work (assuming you are talking instrumentation wiring here)
4) As Mr Humper said/mentioned, tow the outfit to Wayne Maddox Marine in margate, having booked it in first, and leave it with him and insist he does a sea-trial when he has finished the service. He will do this, it'll ensure it all works OK.
5) When you come and collect it let me know and we'll go boating around here, at least if you have a problemm, it's just been serviced and the'll be another boat about. You can then buy me a beer somewhere