Hi James and welcome to the forum.
I'd say the first issue you have here is that Tiffany P2's don't come up for sale very often so it's going to take a little patience.
When one does turn up you need to be sure it's a genuine Tiffany glass on the watch (there were some fakes about a few years back - simple way to test for these is does the logo disappear when you clean the glass with rubbing alcohol).
Some other things to look out for:
1) Is the magnet in the clasp.
2) Does it have a P2 caseback? (there are a lot of watches out there made up from bits and bobs and it sometimes happens that a P2 will have a hamilton or P3 caseback - this doesn't effect functionality but can effect the value of the watch).
3) Has the retaining ring for the caseback been cross threaded? (a real pain)
4) Is the bracelet long enough? (it's not difficult to source extra links but people generally know what to charge).
5) If you buy a working one check that the display is big, bright and that the numbers are made up of small dots. Check pictures of genuine working P2's to see how they should look - this way you won't pay too much for a watch with a frankenstein module in it.
Hope this helps and good luck in finding one.