Front speaker is hard to replace. because of addball size. When I had my
380SE, what I did is put a set of 4" component speakers in the front. I put the 4" round speaker in the stock location and cut a sheetmetal to seal the hole right... that allows mounting the the 4" speaker securely. I placed the tweeter on the door, but you can place the tweeter by your 4" speaker if you have room... or otherwise place them on the dash or the windshield pillar with double sided foam tape.
As for the head unit... if you are integrating with the factory amp, it should be fine. Whatever Crutchfield tells you... should be fine.
The rear speaker should be a direct fit 4x6 speaker... If I were you, I'd pick the speaker without the rectangular plate...even if you do use the one with rectangular plate, all you have to do is widen the hole a bit more and cut where the factory grill smaps into so there is no obstruction.
For me, my entire system was wired up with external amplifier that is in the trunk.
Don't worry about the radio... that is super easy. Pull out the ashtray bin, there is two screws to remove... pull out ashtray assembly... you will have to move the gear shift back temporarily...
Once ashtray assembly is out... two metal tab just need to be held down a bit and slide out the radio.