Well I will give it a go.
First off Headphones.
If you get new gear you will most proberbly get volume enough for your current headphones!
No need to buy new low budget reloop headphones if you already have a pair that works.
Save money and buy a decent pair of headphones later (Senhiser HD25/ Denon HP 1000/700 or Shure)
About Vestax VCI 400 I do not know about the volume but I am no fan of that controller at that price.
I would look at a Denon MC6000 and if you want more knobs add one or two Denon SC2000s or an NI X1.
For the same amount of money you will get higher quality IMO.
And MC6000 I use about 25-30% volume on headphone, 50% in traktor master volume and 40-50% on MC6000 mastervolume channel volume always on 100% (I use only x-fader for mixing) gain on 25-40%
(MC6000 has a real hardware mixer built in so it has its own master volume channel volume and gain
Master volume is separated from software and is not tagged to get better soundquality)
I know Apple has different graphicscards on their laptops so the one you refer as crappy might not be so crappy if you get a Macbook pro with the better graphicscard?. I have an old Macbook pro from 2008 that has no issues at all so I strongly recommend Mac! Because of the issues you wrote about with your PC.
Now I suppose you can not buy all this stuff at once so my recommendation is to first get a new controller.
Then you can still use your PC and headphones.
After that I would get a Mac if I were you.
At last you should get a pair of headphones that last for you whole DJ carrieer so you will never have to buy that again IMO. Then there is Senhiser HD25, Denon HP1000 or HP7000 and Shure HPs These three brands I am convinced has very high quality and lasts for a long time.
For the rest of your questions I am sorry that I can not be of help…..
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