1: What is the Year, Make, and Model of the vehicle.
2: What is your budget?
3: What exactly do you want your outcome to be? (Good sounding music, or loud bass?)
4: And specific sub sizes that you prefer more than others?
5: How much trunk space do you want to use up?
6: What type of features are you looking for in a CD Player (aka Head Unit)? (Ex: USB, AUX, BlueTooth, etc)
7: What type of music do you listen to?
8: Is your stock stereo system amplified? (You may or may not know this)
9: Do you have Steering Wheel Controls? (Volume/Seek on the steering wheel)

1. 1995 Volvo 850 Sedan
2. Sub $500 preferred, cheaper (with quality) is better
3. Good sounding music, low bass
4. No
5.
6. USB and aux inputs
7. Alternative rock mostly
8.
9. No

united states?

Yes, sorry, should have stated.

http://www.sonicelectronix.com/item_63986_Pioneer-DEH-X7500HD.html
http://www.sonicelectronix.com/item_71703_SKAR-Audio-RPX525.html X2
http://www.sonicelectronix.com/item_19279_Belva-Sub-Box-1SV12B-Belva.html
http://www.sonicelectronix.com/item_63983_Pioneer-TS-W310S4.html
http://www.sonicelectronix.com/item_44472_MB-Quart-ONX2.100.html
http://www.sonicelectronix.com/item_45056_Belva-BAK42.html

in case it isn't clear you will need 2 sets of the skar audio speakers.

also how to wire the sub to the amp correctly

Yeah I got that I need 2 sets of speakers. Also, I'm a complete beginner at this (never done it), should I have somebody do it for me or it simple enough to do myself?

remove the battery + wire to avoid any shorts

head unit just follow instructions and google if you need to know how your dash stuff comes off.

speakers again just google how to remove the door panels...another note on this is you might have 6 sets of speaker wires but you will simply be unhooking the ones to the front small speakers and using the other 4.

amplifier wiring works like this
you run the main power cable(fuse removed) through the firewall(wall between your cars interior and the engin***) then remove trim panels along the door bottoms to hide the wire under. when doing this part you will also be running rca+remote wire from the head unit with the power wire to the trunk.

one you have the rca and power wires to the trunk spot you will then be installing the ground wire. you'll need to sand off any paint where you attach the ground wire. so if you use a seat bolt you'll need to remove the paint that is in between the car body and seat bracket as well as the paint on the seat bracket itself. to avoid having to do extra work like that just drill your own hole and run a bolt+nut this way you only have to sand 1 spot.

i have an volvo 850 myself and shouldn't be anything hard with it

you have a hole in the firewall on the right side (left side if you standinfront of the car) than just pull away the plastic all the way back to the trunk (should fit a 2 gauge wire there (i pulled a 0/1 gauge there so i know it wont be a problem.

is it the automatic? if so you should pull the rca cables under midd tunnel

and speaker wires on the left (if you wire them up to a 4 channel amp)