The Supernova rear is amazingly bright, and would recommend, it's trickier to fit and route the cable but once that's done, that's hopefully all you need to do.
I got the battery rear on the Elephants because let face it, it's the kind of bicycles you don't really need to pay attention to.
Actually just a battery powered one, good thing about rear light you can run it for a couple of months before it start to fade.
On the unfinished bike, it's one of this;
If I need brighter one, I fit a rechargeable one on the seat tube (like the Moon Comet).