Hola Paquito
I agree with the lads, building your own makes much more sense (in pure financial terms) once you go beyond the €1000 price tag. It's not rocket science, especially if you have somebody supervising you, and a box of Mahou
Dell/Alienware will give you a 2 year warranty on the whole system I think, but many of their components (motherboard, power supply, etc.) are absolute crap and WILL fail. If you assemble your own and something goes wrong, it may not be that obvious to identify, but once you got it, components have much better warranties (e.g. Corsair PSUs have 7 years, EVGA have lifetime warranties, some SSDs have 5 years....)
Another VERY important factor to keep in mind is that, while you have now a €2K budget, maybe in a few months you land another job and get another €1000 to spend, so you want room to expand - i.e. add more graphic cards, more RAM etc. Pre-made PCs are very difficult to upgrade, possibly don't have expansion slots, not to mention that doing so voids all warranty automatically.
For me it's a no brainer - make your own. We'll be very happy to help!
My suggestion is going for a motherboard based on a Z77 chipset or similar, which will allow you to unlock the integrated graphics of the CPU, drive the monitor with it, and dedicate the TITAN solely to octane render. Make sure the motherboard has at least 3 slots for big graphic cards - don't be afraid to spend €200 or more on it.
Also, a power supply of 750W or more, at least 1kW if you want headroom for a second graphics card. That's another €200.
SSD 128Gb (enough I think) €100, Hard drive as secondary storage + optical drive €100
Processor i5 (with K suffix to overclock) €200
TITAN €1000
16Gb RAM €100 (2x 8Gb sticks, you can another 2 later)
Case and fans €100
Windows 7 retail version (windows 8 is a torture) €100
This makes all €2000, but the numbers are just approximate