Gut everything that is not engine harness related and do it at the same time, you will be kicking yourself for not replacing one wire a year from now.
Once gutted, run a 8 or 10 awg hot and ground up to a distribution panel in the console, then power everything from that. Also add a new switch panel for all your accessories. If you do it right there shouldn't be anything beyond your main power, stern light hot and bilge pump hot running between the console and the stern. Then add a grounding strip in the stern for your engine, bilge, stern lights, etc to clean up back there.
As far as gauges go, you really only need a tach. Here is a pic of mine...
I really only need the tach as the Garmin does voltage and now fuel flow, but I had them already when I went to do the mounting so I kept them.
Dennis is right, Genuinedealz is cheap, I think I got all my stuff through their ebay page. Use marine grade tinned wire and heatshrinks or you will be doing this all for nothing and will be back in there again in two years. As everyone else said, take your time and its not a big deal. I did mine and it is really nice knowing how everything is wired and knowing that when I hit a switch it will work, not that please turn on prayer you do when everything is corroded!
Good luck.
-Svence