You are already in Europe, as far as import taxes and E.U. compliance is concerned. The only changes needed would be for adapting left-hand drive headlamps.
If, however, you are planning to import a vehicle from Croatia or Serbia, this is a matter of which somebody else would be qualified.
I'm on holiday on the Dalmatian coast of Croatia and have seen some clean carriages of which I plan to buy one for export to Germany. This could be a can of worms, since the vehicles must be taxed accordingly and a full inspection will have to be performed. Buying a similar vehicle from Slovenia would circumvent most of the E.U. paperwork and all of the import taxes. However, such a vehicle should carry a higher market price, if the seller was to be aware of the advantages of a buyer choosing his/her vehicle over an identicle vehicle sold in Croatia.
Does anybody know about going through such a procedure?