You can look around for one without researching first - it's all fun. i just bought my first Featherweight and love it. i'm still learning it, and fall back upon my "old" Slant-o-matic 503a, but the next sewing retreat i'm going to take the Featherweight and get more familiar with it. There are a lot of threads on this site about FWs too, complete with lots of good information.
There is the Featherweight 221 that has a flat base and a 222 that had a free-arm that drops down for sewing things like set-in sleeves. i think those are less common. there are lots of 221's around in various conditions. mine is a "centennial" - it has a blue ring around the trademark S plate on the front and was made to celebrate Singer's centennial anniversary. Those are a teeny bit more valuable, if all else is the same. Value seems to vary a lot depending on where you live.
Here's a handful of fun sites that I had bookmarked:
http://www.singer-featherweight.com/index.html
http://jillybejoyful.blogspot.com/20...herweight.html
http://thefeatherweight221factory.com/page7.php
http://www.novamontgomery.com/singer...-the-month.htm
http://www.featherweightfanatics.com/