It is far better to go for the euro4 spec engine rather than the earlier euro3 spec. The euro4 spec engine does not have the massive coking-up problems that the euro3 had, due to a revised camshaft (or camshafts?). The ECU was changed at the same time, so possibly the euro4 engine needs the newer ECU to function correctly. The euro4 engine also got improved piston rings to lessen oil consumption. I think the euro4 engine was introduced in 2004 but maybe 2003.
35-50,000 Km seems extremely low for engines at least 11 years old.
I do not know how to tell the difference between the 2 engine variants just by looking.