Marko, sure it can work but none of us can tell how big you are or how much you plan to carry. 500miles is no big deal. But if you're 200lbs and wanting to carry the kitchen sink on 24spoke rear wheels there are some necessary changes worth considering. The guy at the bike shop was there and provided a worthwhile recomendation. It's not like you have a cheap bike and he's recomending an "expensive" rack(it isn't) he's recomending a rack for someone with a road bike.
My $.02 would be to first determine how little gear you can carry instead of fixing up a road bike to carry as much as possible through a rack and panniers. See if you can get the rear load down to 15lbs on the rack without panniers. 2.5lbs of panniers is a handlful of gear, with a compression sack worth of stuff under the handlebars.