I haven't found a dual purpose switch, but post #1115 has a link to a good switch.

As for a 12v battery meter, check out this link
http://www.helihobby.com/html/measurement_tools.html
The LED indicators for 12v batteries are pretty cheap and easily mounted. PART#HCAP0331 might work well for you. I don't know about waterproofness, but if you used a clear battery box, you could mount it on the inside and put the button some place protected like on the top tube under your seat or something.

Lock & lock clear food storage containers made by Heritage Mercantile are pretty cool battery boxes. They are VERY tight because of a silicone ring on the inside of the lid, but very easy to use because of four locking hinged tabs. I found them at grocery stores, Target, Walmart...for $5 or less. The 1.5-5 cup containers seem to be perfect. Might have to use some flexible poly tubing to make a "snorkel" for ventilation.
http://www.organize-everything.com/lolofostco-all.html
Would probably work with those adjustible PVC-pipe wall hangers. Any home improvement store carries them.

18ga cable would be sufficient, but you'll get less resistance from 16ga. Quality-cased speaker wire would be just about right.

Thought about a rear light? (strobe vs multi-LED)