The should be a direct fit. Honestly, the only reason i'd replace more than one speaker is if you bought upgraded speakers in a PAIR. You want things to sound balanced from left to right.
6.5" SHOULD be the correct size for the front doors. And electronically, it's just two wires. No issues to worry about with the stock headunit.
If you want to do this job yourself... take a look at the DIY Sticky on the main page. There should be something there with instructions on how to remove the door panel. Maybe a window regulator replacement or a speaker install DIY.