If you can't find a replacement PRS-700 and decide you might be willing to use a cover with light, don't forget the M-edge covers with the light that slips in behind the reader. I have one for my PRS-900. I never take it off and have never had to change the battery in over a year(?). Of course, I usually read with an overhead light on.
Good luck! There are no bad choices, but I hope you can find a barely used PRS-700!
P.S. For what it's worth, I read mostly on my Pocket eDGe these days, and when I am trying not to wake my husband I use a flashlight app on the (Android) LCD side to illuminate the e-ink side.