I can't speak for others, but I've had a couple of workshop manuals on CD bought off eBay; both loaded on my then computers, and worked fine. What you get is the full manual for every car in the range between the validity dates - the Mercedes WIS (Workshop Information System). The sellers all claim they have the right to sell them, but I suspect they're all bootleg clones; so long as they work, you may not be too bothered about that.
If you want the parts catalogue, that is the EPC. Some bootleg CD versions have both WIS and EPC, some just WIS.
WIS is an absolute PITA to navigate around and find what you want until you get the hang of it, and if you only use it occasionally, you'll be (re)learning every time. I bought a new computer a year ago, and haven't bothered with loading WIS on to it, but I leave most of the oily stuff to others these days, and for most routine work there's probably a video on YouTube somewhere anyway.
