I have a 2.0
Scenic, not the Grand, but I would imagine that mileage would be similar. I get about 28mpg, which I think is poor compared to my old Scenic, but is apparently about right for the MkII.
The dashboard should be fine now, it was bad batch of circuits. That's what happens when you buy from the cheapest bidder.
Check the mileage, as it would have been reset to zero when the new dash went in.
My panoramic roof doesn't leak, so I would think that your's will be OK once repaired. I would ask HOW they are going to repair it though.
Check for creakiness in the front suspension mounts. Get someone to turn the steering wheel as you feel the top of the spring. The bearing is prone to seizing. It's a common problem, £20 per side, cheap to fix if you do it yourself. Mine is like it and I will get around to fixing it in better weather.
Windows should operate properly and smoothly, not too much to ask but Renault don't seem to be able make ones that do so on a regular basis. Check for odd noises as you operate the windows. Expensive to get fixed, but cheap enough if you DIY, just really tricky to do.
There might be a weird hooting noise from the engine. Renault deny this, but it is recognised in Europe. It's normally a cheap £30 fix, new pipework on the manifold that's all. I'm doing mine tomorrow if it stays dry.
Check the blowers work on hot and cold and at all settings. The resistor pack is prone to melting.
Check the handbrake works well. If you haven't used an automatic brake before it is quite odd, but you get used to it. There is a software recall for faulty units that stick on, check it's been done.
Check that the handsfree works correctly on all doors. It should unlock as you put your hand around the handle, no need to pull. That's if your model has full handsfree of course.
General wear is often higher than normal in MPVs and Renault do not have a good rep for the quality of the interiors. If treated right, they stay good, if neglected, they look rough.
Those are the bad points, after all, you don't want to hear whats good about them. You will see that for yourself.