Usually people that go with coilovers either want adjustability or a very low drop (3+ inches). If you son't need those things and you are driving your car on the street, then springs should do you fine. It sounds like you don't want a very low drop so TRD or Eibach lowering springs might be good for you at around 1.25 inches front and back. Your stock ground clearance is 4.7 inches, so that would give you 3.45 inches of clearance. If you do get springs do the shocks at the same time. KYB GR-2s or TRD shocks will do good for the street at a good price. For a bit more stiffness, the Tociko HP costs a little more.
As for the panel filter idea, it doesn't make much sense. There isn't going to be any room for an airbox inside your wheel well plus a good -sized cone filter has just as much or more surface area for filtration as a panel filter and takes up less space.
Good luck.
PS if you are still interested in coilovers check out groundcontroll. Or if you want to full-out auto-cross the car or something look into the Tein HA Damper system only $1230, full suspension.