Either an old chromo mtn, touring or even road bike that can fit 35 mm or better tires would work well. One chain ring and a wide range of sprockets, say 11/34 will give you gearing for most riding. As the Skipdiver explained, this can be done at the fraction of the cost of a new bike. Your wheel weight and tire choices are critical to ride quality, and it is very nice to have a couple of wheelers with the same rim size so you can change for different types of riding. For example, my Al Cannondale with 850 gram studded snow Nokian tires is a very different ride than the same bike with 1 1/4 inch lightweight slicks. All three of the frame types above will weigh close to the same, the wheels/tires and possibly the chainrings , being the major weight differences. The other thing is fit. A bike with lively wheels that fits you perfectly with no mechanical problems will be a joy to ride, even if a little heavier. Once you have been riding a while you will be better able to judge what type of bike you want.