want to sell a Commodore 64 whith a monitor for around 300kr (1kr = around 7,5 USD) (good price?),
Huh? $2250?!? No way that's a good price!
How can i check if the monitor works whith my Amiga 600HD? Some extra stuff that i need to buy?
Depends on the model. If it's a 1702, 1802 or similar early model forget about it. Those old monitors only have composite video. If it's newer like 1084, 1084s or 2002 etc it will work with an appropriate cable.
Confirm the price though, I think you made a mistake typing! There's no way it's worth $2250 :-o