Many of my relatives came from around Göteborg, and, it may well be that you are some relation. I haven't found any from Varberg, however.
It may interest you to know that there are basically 2 origins of the Dahlgren name in Sweden. One was from Varmland, the other was from Dalen, a farm near Aklinga. My great great grandfather left the farm to become a minister, and so was given the name Dahlgren(meanin 'branch of the family from Dalen'). His family name to that point had been Svensson.
He married a countess, Ulrika Elisabeth von Seth, and was a minister at Vänersnäs, near Vänersborg. His son, Nils Emmanual Constantin followed in his footsteps as a minister at Vänersnäs.
Many torp farmers took the name of their 'master's' family when they left to go to another part of Sweden; this was done in England as well. It was a way of tracing them, to know where they came from if they got into trouble with the authorities. It wasn't done to be underhanded, it was the law. As a result, there are many 'Dahlgren's' in the world who are not from either of the original Dahlgren lines.
I don't know if this is of any help, but I thought you might be interested.