LoL.
They are very strong cars, but as WLDCRD said, not unbreakable.
I wouldn't call them 100% reliable.
Only buy one if you have backup funds for maintenance and repairs.
But once they're up and running, they're usually fine, bar the initial hurdles. If you manage to find one that is already up and running, then you're set.
If you're thinking of spending 20k max, then buy a s2 R33 for less money, you'll have some money left over for mods, plus maintenance and repairs will be a hell of a lot cheaper. But at the end of the day, a GTR is a GTR. So if you can afford it, go for it, you'll never regret it.