JuliusWest :
$1400 - $2000 all up for the PC. More if needed, but not overkill. I havent built a pc before, but I have been told its the way to go. Just finding it hard to put a list of all the components together. My mates are only used to putting together gaming pcs. My research tells me this is a whole new feild
In all honesty, building a Gaming PC and an Audio PC tend to be the same thing. The difference however is in the visuals. In a gaming build, you want to lean towards graphics-based processing while in a sound build, you will obviously focus on an audio-based processing machine.
I would still go with the processor I defined in the beginning along with these parts:
Motherboard: ASUS Z170-A LGA 1151 Intel Z170
Ram: Personal Preference honestly. RAM is very cheap
HDD: Another personal preference, but I would go with 2 SSDs and one large HDD. One SSD for your operating system and one SSD for recording audio to. The HDD would be for large file storage, etc.
PSU: This one is important. I would go for a 750w Gold Corsair or Seasonic. Nothing specific is needed, but please get a quality unit. Without quality power, you could improperly power or even fry your system. If you need more help on this, let me know.
GPU: This is probably where your friends got confused. Something like a GIGABYTE GeForce GT 740 2GB would be more than enough for an audio build.
Sound Card: I am a big audiophile, but sound cards are definitely not my specialty. An External Audio Interface is normally used when you do not have an audio input for your computer, but if you feel that your MIDI Keyboard is well-suited for an input, then a basic or on-motherboard internal soundcard would be just fine. However, if you are looking for a good input, then go with a "Native Instruments Traktor Audio 2". Super small, awesome sound, very cheap.
Let me know what else you need help with and I will do my best.