Is it a good idea to buy a 7-year-old MTB? I'm a single speed rider and I'd like to buy an MTB.
My main purpose for MTB is hardcore forest riding(rocks and little downhills).
I thought of buying an used one and I saw today Marin Rock Springs 2007 at a local shop.
The original price is about 1650 euros($2300) and their call is 500 euros($700)

The shop has pretty good reputation and the bike seems to be in good condition.
Fortunately there is a tiny "mountain" behind the shop so I could testride it and the feeling and the fit were just great. 

Is it a good idea to purchase a 7-year-old full suspension MTB?
Will I struggle with constant repair of breaking components?
Thanks a lot in advance :)
Being the owner of a 2004 dirt jumper, and a 2007 full-suspension, I can say it's just fine. However, be prepared to solve problems...creatively. Certain things that are specific to the bike may not be available anymore, but there's usually an after-market part to fill the gap.

The main reason not to buy an older mountain bike is the difference in how it's made, what it's made of, and what it can ride.

How it's made: Newer carbon, and even alumninum, is much stronger and prepared in different ways.

What it's made of: New steel is generally lighter than old steel frames, this reflects both material and manufacturing changes. Some things like carbon and titanium have come down in price, but are still worth more even on older bikes.

What it can ride: The downhill of yesteryear isn't what it is today. 3" of travel back then was more than enough for some serious trail riding, but now that same style of riding might require 4", 5", or 6" of travel.

However, these are just general problems that can arise or should be taken into consideration. I've had zero trouble with my older bikes (although the full susser is pretty heavy and not the greatest to pedal!). So, I would recommend that test ride and then compare to what a new model would cost and make a decision. Good luck!