Currencies: Belgium and others use the euro
UK uses Sterling
Channel islands use their own version of sterling
Switzerland uses Swiss francs
Norway uses the Norwegian Krone
Sweden uses the Swedish krone
Denmark uses the Danish krone ( I think)
Its another indicator of each countries' history and
culture.
Passports: France I believe has withdrawn temporarly from Shengen, and has a particularly rigourous procedure for GA arrivals and departures. There is also a strict limit on the amount of currency you are allowed to have in your posession.
I am told fuel in Italy can be a problem to obtain at many airfields.
Aircraft documentation and insurance certificates, 'sales tax' paid proof and German noise certificates are all required. Special translations of insurance certificates are required for flight in Spain.
Nothing in all this should put you off your ambition, but to avoid frustration and disapointment plan well ahead and do not expect it to be a breeze.