It it safe to say any phone marketed as a 'IP Phone' will work or are
there gotchas that I need to know about ?
There are ALWAYS gotchas with every manufacturer. The problem is that there is not a comprehensive list of every gotcha for every phone maker because that's just too much work to keep up with. (Things change, firmwares get updated, etc.)
If you want a really inexpensive phone that "just works" then you might want to check out the Yealink T26. Alternatively, the Snom 300 is cheap, but it has a really small display (although it's backlit, so at least you have that.)
Personally, I would stay away from the Cisco 79xx phones unless you really, really enjoy drama.