Repetier host/ slicer best speed settings?

That depends on your layer height, as well as if the model has tight corners and rounds. I usually keep things set at 25 for the smallest and tightest to 40 for internal infill and areas that don't need to be precise. Then other settings fall between those two.

As stated, really depends on everything.  What your print looks like, how fine of detail your wanting, how clean you want exterior perimeters....etc.

25-30 mm/s sec is safe.

I run anywhere from 40-60 mm/s for most of my standard perimeter speeds typically.  I am running an E3D head and a different extruder, so not sure about how comparable it is to your setup.  I've found anything above 60 mm/s will sometimes run into extrusion issues.

Quality difference of the parts at those speeds will depend on your machine.  I've taken most of the slop out of my machine and barely see any quality difference between 30mm/s  and 60mm/s.  But when it was pretty much stock out of the box, I would see degradation of quality as I pushed it faster...particularly on smaller parts and corners.