There is a wealth of info on here regarding the Scenic,but a few good starting points for things to check would be....
The auto gearboxes can be rather fragile.Make sure you road test the car before buying it and make sure it has all the gears and they are smooth to change.
The 1.6 8 valve engine is pretty good,although headgasket problems aren't that unusual.The 8 valve is much more straight foward to work on the 16 valve though,and really there isn't much of a performance difference between the two in the real world.
Broken rear shocks are common,but they are cheap and easy to replace.Listen for any knocking from the rear of the car over bumps.
Thats all I can think of for now,but I'm sure there will be plenty of other advice given to you shortly.
Whilst looking at the forum,please remember that people often only join because they have a problem with their car,so it can give you a negative impression,but in general the Scenic is a good car and there are many people that are very happy with theirs.