most of the hondas is easy to take apart the dash. ive been doin systems for a few years so i know that aftermarket is almost all the time better then stock(unless u have bose or infinity). depends on the model accord youll get i think the style of the dash is diff in the new gen. sometimes when the dash is intergrated w/ the radio the kits tend to be around $100 but normal kits that just fill in the gaps go for around $20 and always get a wire harness usually around $15-$20 it will make your life alot easier.
i like the idea or the aux input on the stock but $450 for an mp3 player that only puts out prob around 35wattsX4 max, compared to aftermarket radios that go up to 60wattsX4 max, its not worth it. the rule of thumb is the more watts:1 the louder it will play and 2 the cleaner the sound will be when played quieter.
another thing alot of people tend to upgrade w/ amplifiers for subs or interiors so most of the time the stock radios dont have rca outputs. i would say to get the stock peice of $h*t n then upgrade later if you plan on doing upgrades on the system.