Check your city's Craigslist offerings, I've seen some great deals flying by on G3 ibooks and PB's
Have cash ready and be ready to jump in the car and go pick 'em up...listers are looking for a quick sale. I usually email them my cell phone # right off the bat, and make it clear I'm serious. Have met some great mac users and had smooth transactions so far with other CL'ers.
Good batteries and decent ram are worth a little extra IMO, and it's nice to have a second machine for travel.
I'll be leaving soon for an extended overseas stay, and just bought a new battery for my Pismo. My 15" Tibook gets delicate treatment, and I didn't want to constantly worry about it getting roughed up. I picked up the Pismo (from Craigslist) for just such occasions, but a G3 500 ibook or better should fit the bill nicely.
Happy travels.