For speakers, are you going to run a 4 channel amp? For a lot of the speakers mentioned here, you'd definitely want a 4 channel to run them. If you don't, the Alpine Type S, Polk DXi, Pioneer A series, or even Infinity Reference speakers would be a great start. To get the most out of most speakers, you'll need an amp. For head units, as said before, Alpine or Pioneer would be a great bet. Kenwood also has some pretty nice models. I also want to throw this out there, I have or currently use everything I mentioned. I'm not just rambling things off here. I've had Alpine and Pioneer head units in my Caprice, Jeep, Jimmy, and S10. I currently have an Alpine 9855 in the Jeep and a Pioneer Premier 800PRS in the Caprice. I ran A series Pioneer speakers in the Jeep before, Type S speakers in the Caprice, Infinity Reference in the Caprice (the 4x6 plate speakers require you to hack up the factory plastic brackets to make them fit), Polks and Alpine Type S in my Jimmy (as well as 2 different Alpines and a Pioneer HU), and an Old Alpine 9807 in my S10. The wife and I ran a Kenwood Excelon 693 in her PT Cruiser until she sold the car in January. We had that in there for a couple years and it was a really nice budget HU.
-94 Caprice Classic LS - system, Gary's wires, 200 amp alt, tint, first base/home plate delete, clear corners w/green parking lights, LTZ hood emblem, SS springs
-97 Grand Cherokee TSi - 4" IRO lift, OEM skids/hooks/hitch, sliders, 31s, rear locker, system, tint, cloth, clear corners, Magnaflow stuff
Refs: DesertDawg, 214Texas, QUICKSSILVER, SSon22s, MadMav, gary burton