I am 182 cm tall. Is a road bike with a frame height 54cm good for me? I have an option to buy a road bike that has the height of the frame 54 cm : https://www.willhaben.at/iad/kaufen-und-verkaufen/sport-sportgeraete/top-rennrad-ktm-strada-nur-9-3-kg-alles-shimano-ultegra-132209154/ . I am 182 cm tall. When I went to a shop and asked the "expert" about the bike, he suggested me to buy the bike with a frame of 56 cm. 

Is there a big difference between the two sizes? Should I buy the bike, or it doesn't fit to my height?
I am 175cm tall. I rode a road bike recently with a 56cm frame. I it had been any bigger I wouldn't have been able to ride it safely. 54 or 52cm would have been a better fit.

I suspect that a 54cm frame would be a bit small for you, but not painfully so. Get a 56 if you can.