Over-planning is the worst thing you could possibly do ever, if you don't believe me just read what these 2016 ralliers say...
"Where's the fun in knowing what to do? Speed head-first into the unknown. That's my motto from now on."
-Mark, Team Should've Bought a Micra 2016
"Take nothing. No food. No clothes. Nothing."
-Lewis, The Yurt Lockers 2016
"Don't get worked up about things, Who you do the rally with is most important, Don't plan too much, its going to take longer than google maps says so. Maps and locals are better guides than any form of GPS"
-Joe, Team Keystone 2016
Wise words there gents, wise words.
The boring, unavoidable paperwork malarkey is really the only planning you should be doing properly. As long as you vaguely know what countries you want to go through, have the visas for these countries and the paperwork you'll need to get through borders, you're good to go. Leave packing until the day before, take joy in plunging into the unknown - it's the whole point.