A. is $172 for head unit+all the install parts+4 speakers fine? i know speakers may feel optional but in reality its best to get them out of the way because aftermarket head units tend to blow factory speakers.
B.
https://www.youtube.com/watch?v=iSbzR9lv2OM is how to remove it
after that you just need to follow the instructions that come with the wiring harness and dash kit. wiring harness is paint by numbers and dash kit slips on.
and don't worry about electricity and being shocked or anything of that because that doesn't happen if you use a wiring harness instead of chopping up the factory wires.
C. you can either go with electrical tape or butt connectors for the wires. electrical tape is easier(don't cheap out on electrical tape if you go this route) this is a good option
http://www.homedepot.com/p/Scotch-Super-33-3-4-in-x-66-ft-Electrical-Tape-6132-BA-10/100073402butt connectors are the better choice as they are faster and are more reliable in the long term but they can also be annoying and fail to hold the wires after you mash them down. you can remedy this buy doing pull test everytime+adding some electrical tape on the ends as well. i'd say do both to avoid having to take it back out ever again to fix something thats came undone.
D. depends on what you mean by "sound system". door speakers generally require popping off door panels after removing a few screws since they are almost entirely held on by tension from
http://c1552172.r72.cf0.rackcdn.com/294945_x800.jpg which just require a firm steady pressure to come undone.
adding amplifier(s) requires running power and ground wires. the power run all the way from your battery to the amp location and the grounds will require some drilling and sanding near the amp location as well.
you have to run rca+remote turn on wires from your head unit to the amp location(for subwoofer amp)
you have to run rca+remoter turn on wires from your head unit to the amp as well as run speaker wire from the amp to all the door speakers(for door speaker amp obv)