Try to plan early, but commit as late as possible. I think totally forgetting about it until 6 months from final is a recipe for a less than what could be optimal outcome.
The sooner you start planning, the more you can optimise / iterate your panel concept, get to know the various avionics options and how they fit together. Also plan to at least look at each option in real life, either flying with someone else or at least getting the "touch and feel" at an event like Oshkosh. There are some aspects about e.g. EFIS's you simply cannot grasp from reading websites and manuals.
rgds, bernie
p.s. we are very happy with our choices, as we went into great detail about ergonomics, placement, functionality etc... it simply delivers the best situational awareness we've ever had in a plane for less $$ than many think. this doesn't say there would be newer equipment in, could we start over planning today. things happen so quickly in the avionics world.