^^This.^^
Something like a 52cm Centurion Iron Man or 19" Schwinn Tempo would be perfect. Light at 21 pounds or so, excellent Shimano 105 12 speed component group, very tough and reliable. Lots of them sitting unused in basements and garages, figure under $300 easy. Downtube shifters, but trust me you will learn to live with (and love) them- you can ride the hoods and easily reach them. All you need is the bike, a set of new tires, and a couple small bottles of hobby shop paint (I prefer Testor's because it reminds me of building models as a child in the 50's) in ugly colors. The less you clean it, the less likely it is to be stolen.