Thanks for the replies so far.
Jan - I like to do a "big airport" trip every so often - you are of course right it is cheaper and easier to choose one of the smaller fields, but that is part of the challenge.
500 and Tu - thanks for this. I have checked out the fees on the website; what I was worried about was the need for an Austrian advisor / VAT number. That would probably be a bridge too far and I'm glad to see my interpretation that under 2T is exempt confirmed.
Thanks for the tip on speed restrictions - yes I suspect 160kt to 4DME will be slightly sporty for the mighty 172! I will see if they can advise of times where a request might be tolerated, but am planning on being told no.
Re taxi - are they happy to provide progressive taxi instructions? Or will they expect us to get it right first time? Luckily Mrs JTN, my co-pilot, also has her PPL so can deal with the comms and writing clearances etc.
Anything else I need to worry about? Anyone else have any tips?