Ive flown a Navion a couple of times in the US a few years back, a friend has one.very comfortable, great visibility, nice handling, particularly for an aircraft with a yoke rather than a stick. Gets in and out of short fields and does nice basic aerobatics, loops and rolls from cruise speed I seem to remember. Lots of room. Probably not great on fuel economy as its quite heavy.
There is a big Navion owners club in the US and I think quite a lot of spares still available. My friend got brand new wings, must have been 15 years ago, they were still available in crates, but I think they are all gone now. He replaced his because of corrosion, so that might be something to look for.
I dont think there has ever been one on the UK register and certifcation will probably be very expensive or virtually impossible, so you would have to either keep it on the French register- or German, there are some there, or put it on the N reg.