Intrepid: The fact that you're asking the question says you're appropriately thoughtful - good start. You can pick up some of the mechanical skills from books. I'm sure others can chime in with a library list for you, try Nigel Calder for marine diesels & electrical wiring; pick up Chapmans piloting and/or Annapolis Book of Seamanship by Rousmaniere (sp?); and learn a bit about how to forecast weather. On your charters you probably learned that life aboard differs from land life because you don't have unlimited power and fresh water so you get used to conserving. For the rest, general mechanical skills and common sense will go a long way. (Things like fiberglass repair, which hopefully you won't have to do very often(!) you can get an idea of just by reading the advice in the West Marine catalog, and asking specific questions here at Sailnet when you have them.)
I'd encourage you to go for the big boat and learn from your marina neighbors as well. GET A SURVEY before you buy; a good surveyor can teach you a lot about your boat also. Good luck and welcome to a cool life!