What features do I need in a stud finder for use on plaster walls?
This stud finder with magnet and level is what I keep on me at all times.  It is the only stud finder that I have and works excellently.  It is strong enough to pick up nails used for crosses most of the time too.  

I have no affiliation with the product - I am just a customer that has bought 6-7.  The magnet is super strong and the pointer + level help.   The only downside is the plastic tip breaks on every one if I drop it, but it still does the trick.  A bonus is that it finds screws, nails, tools, whatever... in hard to see/reach places. I don't know how many times I was up in an attic and dropped something and pulled out the stud finder.