The S.I. client is used to stay updated regarding the installed applications on your computer. It's not necessary to have it installed in order to download MDL, but it's recommended to avoid missing updates for applications in case they lack automatic updates. S.I client works best with apps of this kind.
Secondly, the MDL Estimator application doesn't appear to be available anywhere. I have checked official websites and other related websites that mention about MDL, but no download is possible. To check for all the MDL related applications on database of Software Informer, check the following link: http://software.informer.com/search/mdl