I have purchased an iMac and I'm planning to stop using my PC to work with Lightroom.
All my photos and catalogs are currently stored in an external harddisk attached to the PC, in a Dropbox folder.
My plan is to download locally all the Dropbox data to the iMac as it is, and then simply starting using Lightroom on those catalogs and photos.
I have no synchronization issues since i won't use the PC anymore for Lightroom, and the PC won't have access to Dropbox any longer.
Would that work or should i take other steps to secure that the PC created folders and catalogs will work on iMac?
I don't see what Dropbox is being used for. You could simply move the external HD with the catalog file and folders to the iMac.
Since the drive name is different on a Mac than it was on the PC, you will have to reconnect the photos inside of Lightroom, following these instructions (see Figure 4 and associated text) Adobe Lightroom - Find moved or missing files and folders