Whew, no time with computer for two days. School.
But now, to business:
What about a confirmation box "You need to enable mod XYZ. Do you want to enable it? [YES] [NO]", and if [YES] is clicked but the mod is not found or it is different version than the host's, new box appears. "You don't have version 1.23 of the mod XYZ, do you want to download it from the host? [YES] [NO]". Of course, host could disable the option of downloading that mod.