8 cassette speed works ok with friction shifters but it is a little touchy. I'm doing 7 speed now with friction and it is better. A nine speed cassette will "want" a 9 speed chain and that chain will "want" 9 speed crankset. If you need new wheels for durability, I'd stick with 6 speed and get a Phil Wood rear hub and leave the 126 as is. When you really want to upgrade to 9 or 10 speed, get the whole shebang for 600 bucks and get the rear end spread by a pro.
Regarding lights; Have your new light set up include the ability to charge your GPS.