Airy Fairy, your right it is tricky! If you feel like speaking to the people who have done this already (operator in Scotland springs to mind!) or the UK CAA directly they'll tell you what need's to be looked at specifically.
Generally from an airframe point of view, it's gotta be a stock standard with no off the shelf modifications, unless they're backed up with JAR/EASA approved STC's.
Could I ask why specifically a 92' model? The "Price" I assume makes it a reasonable choice at the moment??
Sound's like a very interesting project and I wish you the best of luck with this.
Is it for private or commercial use?
Would you be looking at this one per chance, at about 1 million Euro and change...
http://www.erikssonaviation.com/pdf/...mphib-1992.pdf
Do you need someone to fly it?....