Most travertine pavers are 3cm thick, which is 1.2" You can get coping from 3cm to 10cm thick depending on how you want the edge to look.
As far as quality goes, there are clean stones that have a small number of holes in them, and there are cheap stones that can look like volcanic rock. Also the cuts/sizing matters. Some stones are cut very nice and fit tight together, while others can contain irregular cuts, which makes laying it a pain.
If you can see the actual pallets of stone before you buy that is always best.
Prep has to do more with where you live and soil conditions. We don't have freeze thaw here so we compact 2 to 4" of AB depending on soil type, and 1" of sand. We do a concrete wedge to hold it all together, like typical pavers.