RE: Pre Requisite Issue - Prior to install SMS Agent
It will want to create a ServicePackFiles in the Windows folder. You could save space by using a switch so you can never uninstall, but I'm not sure if there is a switch to tell it where to put this ServicePackFiles folder. If there is enough to finish the install, you could move that folder after and point the registry to the new location.
I can't see MSDTC affecting anything. But I could be wrong. If I were a paid consultant, I'd look it up to be sure. If it were me at work, I'd just enable it and worry about it after, because as I said, I just don't see how it could be detremental in any way.