What's the reason for wanting a Euro E28?
In general, NA vs. Europe, anything remotely desirable or that has character will cost you accordingly. Start with an equivalent $$$ value, keep the number and just change the sign to €€€ or £££ and that would be a minimum starting point, typically another 20/25% on top of that.
FYI the Umweltplakette business in Germany is much easier for petrol cars. Especially if the car is old enough to be registered as historic, then it's exempt from the whole thing anyway. Expect to pay top $/€ for anything like what you describe, not least because of their "TÜV" standards which does mean cars tend to be in much better shape that what's acceptable in NA.
mobile.de is a standard autotrader type site but a very good one. I imagine specialty and classic dealers list there too, and at least you can do some market research and know where to send your local contact to go see the car. Here is a relevant search result for you, I hope:
BMW 5er Reihe
Get this instead
