Looked for an old HP on the Dutch market place but not much offers, just one and that was sold the same day for over 55 euro. That is more expensive as a new 35s
What would be a good model for EE , must have complex numbers.
That might be a tough combination... The classics don't have native handling of complex numbers. You can program the HP65/67 for this, but then you are looking at more money and the hassles of programming and magnetic cards. The HP67 bizarrely puts shift functions UNDER the keys (not over them, as on every other HP calculator) which makes them annoying to use and error prone. Both models depended on the battery to be in the calculator for voltage regulation...it's a common problem that the card reader motor is damaged because someone tried to use the calculator with the external power adapter without a battery in place.
The HP34C is a late LED model with non-volatile memory, and no doubt can be programmed to do some complex number operations. These are not that rare, but their weak point is a fragile battery cover, so watch for problems there. (Also, I will repeat my complaint that the reciprocal key is a shift function, which is terrible because reciprocal is a commonly used operation in RPN.)
I think you would like the HP 15C very much, even though it doesn't have an LED display. The LCD has very good contrast and is readable under normal light. The newer dot-matrix LCD displays on HP calculators are very hard to read by comparison. It has good complex number support built in, and achieves its power without ruining usability. The old ones are collector items, but HP re-issued this model as a "limited edition" and they were selling until recently on the HP website for about $100. Maybe they ran out, but they are still available on Amazon, I believe, for about the same amount.