I find it odd that canceling the installer at any time means that you have to restart the whole thing, regardless of how for you got. I assume it doesn't start installing anything until all the downloading is done.
I got to the point where it told me that my FS1Port extra were old/non existent, would I like to download? I said yes, and a little while into it I canceled, because I realized that the FS1Port extras were the .mve's which I already had. I canceled it and figured I could run it again and skip over everything I'd already downloaded, but this wasn't the case.
So I guess two things. What about having the installer search for already downloaded core files (or installed right after they're downloaded), so that canceling the downloader doesn't mean you start from square one again. Also, having it search more than one directory for the required files. I'm not sure the best way to do this, but all my FS2 .mve's are in my root game directory, and all my FS1Port .mve's are in my fspor\data directory, and the installer told me that both of them dont exist.