Europe is a big place and has a number of different regulations. However I would agree with the advice to sell the tanks and buy new when you get here. The neck thread in European countries is M25 and so getting your tanks retested will be more difficult but not impossible. Also nearly all European countries will need a hydro test that they recognise so you will almost certainly be looking at a test before you get your first fill.
Steel is the normal choice for backgas and Ali for deco/stage/sidemount. Your US spec tanks will have little secondhand value. Tank testing, O2 cleaning and all matters tank are "problematical" here, with some shops having strong, but incorrect opinions about what can or cannot be done or filled. However once you have the right choice for wherever you are going to live it is easy enough. It's also pretty easy to rent a tank if you need to, mostly they come full and return empty - a good system if you don't dive often.
Hope you enjoy it here - lots of good diving to be had.