In the UK at least, amateur built aircraft must be genuinely amateur built to qualify for a permit to fly under the PFA scheme.
I would have thought it would be a much cheaper proposition to buy a good, low hour example locally and do the upgrade. A Dynon EFIS can be installed in a day or two.
Probably best to consult with your national organisations for the legalities.
Editted to add. JAR certification is a non-starter for a non-factory built example of a non-type certificated aircraft.
An EASA issued permit to fly would allow free movement in Europe, but I don't know if it would be possible to obtain one.