I'm not sure if you really need all 4 CDs, I don't think I used discs 3 and 4 for my installation of 10.1. They might just contain extra stuff.
For the md5 file, it should be just a plain text file that contains the MD5 sum of the relevant ISO. You can use MD5summer
to generate the MD5 sum for each ISO and then compare that with the one given in the file. If they don't match, you'll need to re-download the file.
Not sure about the .asc file, as I've never used those.