Hello JABC,
Here are the serial numbers so you can date a machine
http://www.singerco.com/support/mach.../single-letter
And here is an excellent website for you to go to so you can recognise models by looking at them
http://www.singersewinginfo.co.uk
If you only intend to use a treadle for piecing the size is not so important because you will be able to place the bulk of your work to the left - whereas with quilting you need plenty of space to the right. You will do just as well with a model 99.
The general principle when buying a treadle is that any treadle will do anything you want to do with a straight stitch, and with a class 15 you have the added bonus of free motion work. First and foremost look for a clean machine in good working condition. That is more important than anything else. Considerations about models, long bobbins or round bobbins etc. etc. take second place.