Symptoms like this one are often traceable to a faulty stepper-motor which is a small electric motor that controls the position of the car’s throttle and, therefore, the speed at which it idles. If this little electric motor goes awry, the idle speed can stay too high, which is precisely the symptom you’re reporting.

The other likely culprit is a vacuum leak from somewhere on the inlet side of the engine which is allowing too much air into the engine and causing the idle speed to increase. Check for split hoses around the engine bay and don’t forget to check the plumbing associated with the Exhaust Gas Recirculation (EGR) valve which can also crack and allow excess air into the intake manifold.

They are very different cars, the Viva is small, the Falcon big, so it comes down to what you want from them. The Viva is newer and will have done fewer kilometres, the Falcon is getting old and is likely to have done lots of kilometres. That said, Falcons tend to be relatively reliable at high kilometres, so if you are after reliability that's probably the better bet for you.

Regardless of the engine work, the Viva is not a good car. For similar money, in a small wagon, I'd go every time for a Mitsubishi Lancer.