The only textures which will likely changes are:
A-Terrain, as this will be made new.
B-Plants for terrain, trees/bushes etc.. as these will be added new.
All current models will use the same textures and meshes as they do now, as we CAN NOT remake them to a higher resolution/poly count. We have nobody to remake the 1000s of textures and meshes currently in game.
Upon deciding to try the move to UE4 everyone in the team said they would like to be able to ramp up the resolution of the whole game but it is currently an impossible task. A dozen artists would have to suddenly appear who all work for ages remaking everything before we could event contemplate it. The decision to move to UE4 was not primarily taken from the POV of improving the art, there are lots of advantages moving to UE4 has beyond 'looking pretty'.
Now, saying that you will see differences in the textures in PS as we take advantage of new advanced shaders, but as the resolution is very low the quality of the final art will still be limited by that.
An example is this,
https://www.dropbox.com/s/8kwhtc718na76sc/stonesside.png?dl=0 , the texture here is only 128*128 which is why it doesn't look as good as it might. But i still think its an improvement.
Now I don't really know what you mean by 'repository of reference textures and styles' but I'm pretty sure the answer is 'No'. Just use the art in game as a reference to what style PS uses. Its all available in the 'materials.zip' file in the installation directory.
