We have got onto the terraces fairly late some years, 2pm ish, due to England rugby matches, its been a bit of a scrum but managed it.
If you want to experience the build up get up there about 11 am. If you want to see the support race and warm up about 9 am, or a bit earlier.
You're going to be there Wednesday, no need to panic or rush, practice is earlier, from about 6, check CA guide, but qualy doesn't start till 10 pm.
Chill and enjoy.
Entrance tickets sold at, main gate, entrance to Maison Blanche, that's half way between the Porsche curves and the main entrance, and also at the entrance by Karting Nord and Tetre Rouge, not certain of the last two, can someone confirm.
Most of all you will never see or do every thing in one visit, this will be my 14th visit, and I still haven't done/seen everything I want to, plan your priorities, don't try to do to much, just enjoy.