Your line of thinking is too "Smogon". Imprison doesn't do much to Gengar since it has two options to body you with, one of which is Sludge Bomb + you're slower than Gengar anyway. For bulk, opt for 252 HP/120 or more Def/Sp.Atk investment/Sp.Def investment if need be. I personally use 252 HP/192 Def/66 Sp.Def to make Garchomp's EQ a 3HKO, which is pretty neat to have because it also survives quite a bit of other physical attacks, especially with Intimidate support. The extra 66 Sp.Def is optional, and personally, it's so that the mega form can survive even a Modest Mega Gengar Sludge Bomb. Barely, but it does. Only time I lose that match-up is if I take some form of chip damage before I hit it with Psyshock, or if it's running 252 HP/252 Def, which is what I think the Perish Trap users use.
Tangent aside, for moves, it depends on your hold item. If it's choice, go Fairy/Psychic/Energy Ball/Shadow Ball, but then you'd need a different EV spread. If it's something else, go Fairy/Psychic/Will-o-Wisp/Protect.
For future EV spreads, you need to think of meta-game relevant threats, and how your pokemon may deal with them. If it's too much hassle, then don't worry about it, and know that when that threat comes up, you need to switch or Protect and cover it with your partner. If it's possible to help the situation via EVs, do that.