A Windows version of Alldata may be downloaded and used for free as explained here:
http://www.benzworld.org/forums/r129-sl-class/1363762-alldata-mercedes-benz-download-details.html. I installed it on my PC. The download is slow and it took quite some time.
For someone who is not on a tight budget and who is going to be accessing manuals often I still would recommend the $80 DVD from Mercedes. Compared to this Windows version of Alldata the DVD schematics are much easier to use and read. The DVD is better organized too, I think. Also, much of the Alldata content looks as if it has been poorly scanned from M-B documents. The attached screenshot is typical. But the Alldata app has good technical coverage and can adequately meet anyone's needs.