If your friend in London is able to predict the local weather at the other end of Great Britain with such amazing accuracy 7 months ahead, they are very talented. Exactly what year does your friend predict there will be good weather in Scotland?
Look, if this is the kind of nonsense your friend is telling you to base your travel plans on, my advise is to be skeptical of anything your friend says.
It's often rainy in Scotland. Even in the summer. It's a place that's is known for (what many people would consider) rainy weather. You should be prepared for that. Yet somehow, every summer lots of people go to Scotland and live to tell about it, even enjoy it.
Scotland is a big place. You might want to be a little more specific about your plans - where you hope to go, what interests you. If you are just going to be in the larger cities, no need for a car. If you want to get out and see much of the countryside (it's beautiful - although yes, it often rains), then a car is great, many would say a necessity.
Floods can happen anywhere, anytime, and unless there's been a revolution in weather forecasting, they can't be predicted accurately other than in general terms. Local flooding may or may not have any impact on specific roads - in Scotland, or in most places where rain falls from the sky. It seems really crazy to me to let some vague prediction of flooding this year dissuade you from going if you want to visit.
Regardless of your friend's weather forecasting abilities, if you go to Scotland, expect some rain. Maybe a little, maybe a lot. If you're there for very long, you're going to get some rain - count on it. Bring appropriate clothing & footwear, and if it rains very hard take it easy driving. You'll be fine.