1tb is overkill for me, i may have 400 gigs of data that i care about total. a good formula is to take the amount of space you need for the stuff you currently have, and double it. and then add about a quarter of that for the os (for each if you dual boot). which with 400gb of data that is exactly 1gb. then you look at hard drives and try to find a sweet spot in the gb to price ratio that is close to where you want it. sometimes drives of a certain capacity are more expensive than of another capacity, when you look at the current price per gb they need to be. and going up or down in size slightly saves you some cash.
i personally would rather go for a really fast but not all that big drive for your os, like an ssd or a 10k rpm drive. and then a big bulk storage drive for data. but what i ultimately end up doing is just get one drive, and partition it about 20/80 with the os partition being the 20 and the data partition being the 80, and then put in one of my older drives as a backup drive. reguardless do pay attention to write speed, cache size and random access times. to make sure that $5 off isnt coming out of your specs.
and for 500-700 euros you could get a pretty good machine. i built this machine last year for 800 usd (~625 euros). and its way better than the specs you list. thats an i5, 8gb ram, and a geforce 560. mind you that was a year ago. you should be able to get an i7, 12 gigs and a much nicer video card with that money (im just assuming, i havent actually loooked into it because im not planning to build a computer this year). just build a new comp. thats my 2 cents.