It's almost a certainty that you'll be able to use a mouse and keyboard on the PS4--Your PS3 already supports them (though you may need to restart the console after hooking them up). Hopefully ZOS integrates support for them to be useful for more than chatting into the console versions, but if that's not the case, it wouldn't hurt the experience too much--The control scheme should be fairly well-optimized for using a controller. The only reason most PS3 games don't support the keyboard and mouse is because the developers don't take advantage of the fact that they can support it without requiring it. When I play GTA4, I use a real keyboard at the ingame computers--It's much faster than moving a cursor around to type.
Save $1500? As in you were prepared to drop nearly $2000 on a new PC? You'd only need to spend even half that much if you were trying to get to the top end of what's available for PC hardware, and that would assume that whatever PC you currently have is so old that almost no newer hardware will work with it. That's not even getting into the fact that prebuilt systems (even customized ones) charge way to much for what's basically plugging things in (PC assembly really is that easy, all you need are eyes, hands, and a small screwdriver). If you're looking to acquire a gaming PC, buy the parts and either build it yourself, or find a friend willing to do it for the cost of a pizza.