Unless you need to travel building a custom PC is by far the best option. You’ll have a much more powerful and much more quiet computer compared to any laptop out there. Also Apple charges a ton more for their hardware compared to what you’ll find in any retail or online. (Eg: 16GB RAM: Apple - 400$, Amazon 60$). At this point I’d say that we are not talking about a healthy retail margin, but a rip off. I see the advantage of the hardware and software integration at Apple, but at that price?! Also you’ll not even get the newest technology (old CPU socket, DDR3 RAM instead of DDR4, etc.)
In a custom PC you’ll want the following parts:
CPU: Most important part. You’ll have to put most money into this. The CPU is the most deciding part in terms of speed for music production. I’d probably go with a i7-4790K here. It’s not the newest line (6XXX), but it is superb in price/value ratio. Going for this slightly older model means that you won’t get the newest socket, no DDR4 RAM and “only” 32GB RAM max, BUT this will still keep you going for years! Also you’d have to consider that going for an equal or slightly better computer from the newest line (Skylake/DDR4) means that you’ll get a lot less for your money now.
Motherboard: You don’t need anything fancy here. It’ll mainly have to be reliable and have the right socket for your CPU.
RAM: You’ll probably want 16GB. 8 (or 12) could be enough, but RAM is not that expensive and at least you won’t have to worry about reaching the limit even with huuuge projects. As mentioned even going with DDR3 you’ll with most motherboards be able to upgrade to 32GB later so get 2x8GB instead of 4x4GB.
GPU: The choice of the grafics card really depends on what kind of programs you want to use it with. Ableton, Traktor etc don’t need any fancy GPU - in fact you could even use the internal CPU of the processor (included in at least all modern Intel CPUs). You could also just start out without a dedicated GPU and add it later, which is what I would do in your situation: See if it is enough first. If you need a dedicated GPU you could either get a GTX730 or GTX750Ti. The later being twice as expensive and also much more powerful. Again: You don’t need this for audio at all. You might want to look for a passively cooled solution here to reduce fan noise (eg. GTX750TI KalmX by Palit, which is the card I have. Than again I use this for 3D modeling, photoshop and more too)
For storage a 256GB SSD is a good starting point usually, if you have some huge sample libraries (and a bigger budget) you can get 500 or even 1000GB. If the budget is an issue, but you need storage, I’d recommend getting a 128GB SSD for the operating system and programs and another 1TB (7200rpm!) HD for everything else.
Than there is the case (I can recommend a silent double shell case, but any will do unless you get a fancy tiny with too low space. It’s just a matter of slight noise reduction.) the PSU (Getting an efficient 80 Gold+ means you’ll have less heat = more quiet. 600W is enough and a good pricepoint) and some fans (Don’t get the cheapest take some more silent ones).
That’s pretty much it. If you want me to give you a complete build with every part you need to buy just give me a budget and let me know some preferences like: “cheaper or more quiet” “super-upgrade-able or more powerful” "with or without a GPU (heavy grafics, video editing, video games).
Here is an approximate list of what I would get as a minimum (minus the graphics card!): http://pcpartpicker.com/p/yCVRjX
Also that site is great for building a PC and looking for components/alternatives!