+1 to all of the above.
Higher end models regardless of brand have more likelihood of holding value. You just have to know what you are looking at.
The first decision for a decent candidate for a flip (or a keeper) is usually if the frame material has some quality to it.
Read this for a little primer on tubing info.
And remember that in it's original form, a older multi-geared ride has a wider potential buying audience than a SS/FG one. Unless you know what you are doing re conversions for fun and profit, let someone else carve them up and paint them.