Ableton Live 9 has very light requirements, so it is definitely possible to bring the cost down without sacrificing much performance, if at all. Is Amazon.de an option for you? PCPartPicker also has a German price database, if that works for you. I'll mostly keep to the website you mentioned, it has a PC configurator which seems very well detailed and easy to use, even though I don't understand the language.
I didn't see a way to share or export the alternate.be shopping cart list, so
here it is in screenshot form. It comes out to 1.637,38 and I am not sure if that includes tax or not.
Here's a similar build on PCPartPicker but from the German price database, I just changed some parts where I found cheaper prices. I do not know about taxes either, I am not sure how it works in the EU. It may or may not be included in the prices and there may be shipping deals if you were to purchase an order from these shops. It does seem a bit cheaper to order from German retailers, although I don't know about shipping costs and time.
CPU: Intel Core i5-4690K 3.5GHz Quad-Core Processor (242.88 @ Amazon Deutschland)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler (32.99 @ Amazon Deutschland)
Motherboard: Gigabyte GA-Z97X-SLI ATX LGA1150 Motherboard (109.90 @ Caseking)
Memory: Crucial Ballistix Sport XT 16GB (2 x 8GB) DDR3-1866 Memory (140.59 @ Home of Hardware DE)
Storage: Crucial MX100 256GB 2.5" Solid State Drive (107.68 @ Amazon Deutschland)
Storage: Toshiba 2TB 3.5" 7200RPM Internal Hard Drive (74.90 @ Caseking)
Case: Fractal Design Define R5 (Black) ATX Mid Tower Case (118.36 @ Amazon Deutschland)
Power Supply: Super Flower Golden Green HX 750W 80+ Gold Certified ATX Power Supply (89.90 @ Caseking)
Optical Drive: Samsung SH-222BB/BEBE DVD/CD Writer (19.95 @ Amazon Deutschland)
Other: MSI GeForce GTX 980 4GB Twin Frozr Video Card (575.00)
Total: 1512.15
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-01-29 07:39 CET+0100
Now, on to your questions:
1. It's not too hard to build a PC. You yourself said that you know most of the components and can find your way around. Nearly all of the parts will have a manual you can refer to so you can figure out how to install it. If not, there are many online videos you can refer to for help, or you can ask us.
2. The Fractal Define R5 is a very nice case with sound-muffling features. I recommend the GTX 970 and 980 for a graphics card with a silent fan feature when at low temperature. MSI, Asus, and some other models have that silent fan mode.
3. The memory voltage is not a big concern. There are few to no memory modules that run at 2400MHz at low voltage, anyway. It will still be compatible and run fine. It won't matter, I will recommend different RAM for you.
4. There are better quality power supplies for the same price, so I wouldn't be so set on getting that Corsair model. For example, 750 watt Seasonic and Super Flower models are available at the same price as that Cooler Master model.
Edit: You are interested in silence, maybe the EVGA Supernova G2 750 watt model is for you, it has an "eco fan" mode that lowers fan noise.
It is 134,90 at alternate.be. The Seasonic M12II Evo power supply I picked also has "smart & silent fan control" but I do not know how it compares to the EVGA model.
I have a few last things to say, mostly concerning the budget.
Although you can afford the GTX 980 in your budget, it is not the most cost-effective solution. The GTX 970 is much cheaper, also capable of silent fan modes, and will meet your needs at 1080p. However, Nvidia recently had to acknowledge that the GTX 970 had a problem with the hardware design that did not allow it to effectively utilize all of its graphics memory. The GTX 970's issues won't really affect you at 1080p but it could be a problem in the future when games require more graphics memory, so I guess the GTX 980 will work for you despite costing about 600.. 200 more than the GTX 970. The GTX 980 is the best-performing card you can buy and the MSI model I chose has an automatic silent setting for the fans.
You don't have to worry about pairing the same brand motherboard with the same brand graphics card. There is no bonus or added performance to having both, except maybe if they have the same looks and you want it to look the same.
The 4690K and Z97 motherboard I recommended have the ability to overclock. Even if you do not currently plan to overclock, having the option available a few years down the road is nice - overclocking is much more simple than it was back when you bought your old i5 760, there are many guides out there. Until you do change the settings to overclock, the processor will just run normally.
If you do opt for non-overclocking parts to save money, I recommend the
i5 4590 or
4690 (both non-K models) and
a cheaper motherboard from this page like the
MSI H97 PC Mate. You will save around 50-75 by going with those parts.