Which Pulley set?
 

Which pulley set has the best price/quality ratio?

Several suppliers offer these sets with belt and 2,3 or 4 pulleys.
My belt needs changing so it's interesting to change the pulley's in the meantime.

I noticed big differences in princing

Greddy: 2pulleys about 159$??

Unorthodox: 4 pulley set 650$??

Which are crucial, eny remarks what to look for?

engine has 37.000 miles

I don't really recommend buying pullies unless you constantly drive at high RPM's. You can spend your money in a better way elsewhere. The pullies just underdrive the accessories so they don't cavitate when they spin so fast.

OK,

Does it maken any sense buying the Greddy Belt to replace the OEM one, because it's ready for changing.

Is the GReddy belt compatible with the stock pulleys?

I can't think of any good reason to buy an aftermarket belt, and yes it may not even fit. The only pulley I would recommend is the dual alternator pulley if you remove the air pump. The only problem with dual pullies is that it is hard to find 2 belts that are exactly the same size.