And with my qualifications in mind, i am aiming for at least 10 000zl/month. Am i aiming too high?
That's a long shot. Entry level engineering jobs generally pay 5000 or 6000 PLN. 10,000 PLN usually requires at least some managerial or technical specialist experience. You could always ask and see if they bite, though. Since that is your rock-bottom limit, there is not reason not to ask for that much. They're still getting you for less than half of what you would receive in Germany, so there is no need to be shy about naming your price.
As for the girlfriend, it's a story I've seen played out time and time again during my twelve years in Poland, and it's a common theme on this forum, ad nauseam. Polish girlfriend misses her family and wants naive foreign boyfriend to move back with her, telling him that he can easily find work there. It practically never works out for a whole bunch of reasons: He can't find a good job, he can't manage living in close proximity to the "in-laws", he's socially isolated and dependent because he can't speak the language.... and so on. Usually, the relationship ends on a sour note. I've had too many guys crying on my shoulder over nonsense like this.
If you think you can be one of the exceptions, knock yourself out. You might been because at least you have some saleable qualifications in contrast to most of the poor suckers who fall for this line. And a big plus is that you can cut and run back to a well paid job in Germany if things go sour. Another big plus is with 10,000 PLN a month you won't have to put up with any BS from the in-laws.
One last thing: find that job BEFORE you move. Don't count on finding one after you move.