From my experience make sure you give yourself plenty of time, once you start looking at your wiring closely you will find lots of jobs you didnt think you had to do as you end up finding things that need attention to the wiring as your doing it.
And buy quality wire and fixings, plus a quality wire crimper which can save lots of time doing connections if your not going to solder everything. A qulaity crimper for the battery terminals and also any other smaller type of terminal connectors, well worth the investment in the long run.
Instruments usually powered from engine loom or control box not battery directly.
+1 wiring to the gauges / controls etc are normally part of the engine wiring loom, so maybe need a bit of tidying up, but hopefully not re-wiring as that's a BIG job! but wiring to radio, lights etc is easy enough, as the others have said get good kit n draw out the wiring before you start, n take your time
__________________ Member of S.A.B.S. (Wirral Division)
Sula
Thunderbird
Top Banana
wiring to the gauges / controls etc are normally part of the engine wiring loom, so maybe need a bit of tidying up, but hopefully not re-wiring as that's a BIG job! but wiring to radio, lights etc is easy enough, as the others have said get good kit n draw out the wiring before you start, n take your time 
